Japanese Sex-Related Terms

A lot of manga and anime feature sex-related jokes, or dirty jokes, "low jokes," shimoneta 下ネタ, besides those that are simply sexual in nature, like of the ecchi and hentai genres. So, for reference, a list of sex-related Japanese vocabulary.

Anatomy

Let's start with the technical terms for sex-related anatomy. This is mostly the NSFW continuation of the full article about Body Parts.

The words for the genitalia in Japanese:

seiki
性器
Sex organ.
- sei~ - "sexual."
- ~ki - "organ," "device, "instrument."
danseiki
男性器
Male sex organ.
- dansei - "male," "men."
joseiki
女性器
Female sex organ.
- josei - "female," "women."
naiseiki
内性器
Internal sex organ.
gaiseiki
外性器
External sex organ.
seishoku
生殖
Reproduction.
seishokuki
生殖器
Reproductive organ. Genital.
inbu
陰部
Genital area.
- in 陰 means "unseen" and bu 部 means "part." So the term is literally "private parts."

Parts of the male sexual organ:

inkei
陰茎
Penis.
- Also referred to by the katakanization:
- penisu
  ペニス
  "Penis."
kitou 亀頭
inkei kitou 陰茎亀頭
Glans. Glans penis.
kari-kubi カリ首
kari カリ
inkei kitoukan 陰茎亀頭冠
inkei kitoukou 陰茎亀頭溝
Penis crown. Corona of glans penis.
inkei houhi 陰茎包皮
houhi 包皮
Foreskin. (male only.)
Prepuce. (both sexes.)
- hifu
  皮膚
  Skin.
- kawa
  皮
  Skin. Hide. Pelt.
inkei shoutai
陰茎小帯
Penile frenulum.
inkei kyaku
陰茎脚
Crus of penis.
- Since ashi 脚 means "legs," this is literally the "penis' legs."
inkei wana jintai
陰茎ワナ靭帯
Fundiform ligament of the penis.
seinou
精嚢
Seminal vesicle. Seminal glands.
shaseikan
射精管
Ejaculatory duct.
zenritsusen
前立腺
Prostate gland.
- "Prostate" comes from "pro histanai" in Latin, which means "to stand," tatsu 立つ, "before," mae 前.^{(dartmouth.edu:Etymology of Abdominal Visceral Terms)}
nyoudou kyuusen 尿道球腺
kaupaasen カウパー腺
Bulbourethral gland. "Cowper's gland." "Urine path ball gland."
seikan 精管
yuseikan 輸精管
Seminal duct. Vas deferens. Ductus deferens.
seikan boudaibu
精管膨大部
Ampulla of ductus deferens.
seisou
精巣
Testicle.
- Contrary to popular belief, semen isn't stored in the testicles.
- Sperm is stored in the testicles.
- Semen is 5% sperm, 95% other stuff.
- Pee isn't stored in the testicles either.
kougan
睾丸
Testicle.
- Another term for testicle.
- Note that maru 丸 means "round," while su 巣 means "nest," so you could say one comes from its form and the other from its function. So, for example, toward animals whose testicles don't resemble balls, kougan wouldn't be used.
in'nou
陰嚢
Scrotum.
seisou shouyou
精巣小葉
Lobules of testis.
seisoumou
精巣網
Rete testis.
seisou joutai
精巣上体
Epididymis.
seisou yushutsukan
精巣輸出管
Efferent ducts.
seisaikan
精細管
Seminiferous tubule.
chuujinkan 中腎管
whorufukan ウォルフ管
Mesonephric duct. "Walffian duct"

Parts of the female sexual organ:

gaiinbu 外陰部
gaiin 外陰
"External genital area." Vulva.
- baruba バルバ
  varuva ヴァルヴァ
  "Vulva." (generally not used.)
inretsu
陰裂
Pudendal cleft.
inkaku 陰核
intei 陰梃
Clitoris.
- kuritorisu
  クリトリス
  "Clitoris."
- inkaku houhi
  陰核包皮
  Clitoral hood. "Clitoral prepuce."
- inkaku kitou
  陰核亀頭
  Clitoral glans.
- inkaku shoutai
  陰核小帯
  Clitoral frenulum.
inshin
陰唇
Labia.
- shouinshin
  小陰唇
  Labia minora.
- daiinshin
  大陰唇
  Labia majora.
zenteikyuu
前庭球
Vestibular bulb. Clitoral bulbs
inkakukyaku
陰核脚
Crus of clitoris. "Clitoris' legs."
chitsu 膣
shitsu 腟
Vagina.
- The "vagina" is an internal tube connecting the uterus to the outside of the body.
- In English, a lot of people use the term "vagina" to refer to the vulva, which is technically not the same thing as the vagina.
- vagina ヴァギナ
  wagina ワギナ
  "Vagina."
chitsukou
膣口
Ostium of the vagina. "Vaginal opening."
shojomaku
処女膜
Hymen.
- The word shojo 処女 means a female "virgin," and maku 膜 means a "membrane." So this term means literally "virgin membrane."
- Despite popular belief held in dozens of countries worldwide and through history, a virgin doesn't necessarily have an intact hymen: it can tear and rip apart for countless reasons.
- Of course 21st century Japanese doctors know this.
- Even so, shojomaku is still the formal term for "hymen" in Japanese. It's not a slang.
- By the way, "hymen" comes from the Greek humēn, which means "membrane," and has nothing to do with virgins.
chitsuengai
膣円蓋
Vaginal fornix.
shikyuu
子宮
Uterus. Womb.
- For the "womb tattoo" trope, see inmon 淫紋, "lewd crest."
shikyuukei 子宮頸 (or 子宮頚)
shikyuukeibu 子宮頸部 (or 子宮頚部)
Cervix. Cervix uteri. "Uterus neck (region)."
- gaishikyuukou
  外子宮口
  External os. "External uterine orifice."
- nashikyuukou
  内子宮口
  Internal os. "Internal uterine orifice."
- shikyuu chitsubu
  子宮腟部
  Vaginal portion of the cervix. Ectocervix.
shikyuutai
子宮体
Corpus. "Body of the uterus."
- shikyuukuu
  子宮腔
  Uterine cavity.
- shikyuutei
  子宮底
  Fundus. "Bottom of the uterus." (this is actually at the top of the uterus, in the same sense the "bottom" of the barrel is actually at the top of the barrel if it's upside-down.)
shikyuuheki 子宮壁
shikyuu no kabe 子宮の壁
Uterine wall.
- shikyuu naimaku 子宮内膜
  Endometrium. "Uterus internal membrane."
- shikyuu kinsou 子宮筋層
  Myometrium. "Uterus muscle layer."
- shikyuu shoumaku 子宮漿膜
  shikyuu gaimaku 子宮外膜
  Perimetrium. "Uterus serous/external membrane."
yurankan 輸卵管
rankan 卵管
Fallopian tube. "Oviducts." Uterine tubes. Salpinges.
- rankou boudaibu 卵管膨大部
  Ampulla of the uterine tube.
- rankan routo 卵管漏斗
  Infundibulum of the uterine tube.
- rankansai 卵管采
  Fimbria of the uterine tube.
ransou
卵巣
Ovary.
jii-supotto
Ｇスポット
G-spot. Gräfenberg spot.
buroturinsen
バルトリン腺
Bartholin's gland. Greater vestibular glands.
sukiinsen スキーン腺
shouzenteisen 小前庭腺
bounyoudousen 傍尿道腺
Skene's gland

Shared anatomy:

nyoudou
尿道
Urethra.
- Literally "urine," nyou 尿, "path," michi 道.
nyoudoukou
尿道口
"Urethral opening." External urethral orifice. Urinary meatus.
boukou
膀胱
Urinary bladder.
kaimentai
海綿体
Corpus cavernosum.
Corpus spongiosum. "Sponge body."
shoutai
小帯
Frenulum. "Small band."

Nearby anatomy:

kokan 股間
mata 股
Crotch.
inmou 陰毛
chimou 恥毛
Pubic hair.
seimou
性毛
Body hair. "Sex hair." (hair that develops after puberty.)
Androgenic hair. (from androgen, the hormone.)
shiri 尻
denbu 臀部
Buttocks.
- oshiri
  お尻
  Buttocks.^{(polite prefix.)}
ein
会陰
Perineum.
chikyuu 恥丘
viinasu no oka ヴィーナスの丘
Pubic mound. Mons pubis.
Mons Venus. "Mound of Venus."
- Pudendum originates in a Latin word for "shame," which is haji 恥 in Japanese. Usually in the sense of "cover your shame."^{(dartmouth.edu:Etymology of Abdominal Visceral Terms)}
koumon
肛門
Anus.
- koumon katsuyaku-kin
  肛門括約筋
  Anal sphincter. (muscle.)
chokuchou
直腸
Rectum.
esu-jou kecchou Ｓ状結腸
esu-ji kecchou Ｓ字結腸
Sigmoid colon. "S-shaped" colon. "S-letter" colon.

Not nearby, but relevant nonetheless:

mune
胸
Chest.
chibusa
乳房
Breast.
nyuurin
乳輪
Areola.
chikubi
乳首
Nipple.
nyuutou
乳頭
Teat.

Colloquialisms

Japanese, like any other language in the world, also has a number of non-technical terms, colloquialisms, slangs, metonyms, vulgar words, and profanities that refer to body parts, specially the genitals.

zokugo
俗語
Colloquialism. Slang.
ingo
隠語
Jargon. Slang.
higo
卑語
Swear words.
surangu
スラング
Slang.

The most important of such words are the ones that actually have some completely normal and innocent meaning, since those are the ones found in most dirty jokes in anime.

Let's start with the most basic words:

asoko
あそこ
"There." May refer to the genital area, down there, private parts, etc.

nyoudou
尿道
Urethra. "Urine path." (technical.)
- nyou
  尿
  Urine.
oshikko no ana
おしっこの穴
"Pee hole."
- oshikko
  おしっこ
  Pee.

Breasts

chibusa
乳房
Breasts. (technical.)
mune
胸
Chest.
chichi
乳
This word technically means "milk," but can also refer to "breasts."
oppai
おっぱい
"Maternal milk," specifically in the sense of breastfeeding babies, but can also mean "breasts," "boobs," "tits," etc.
dekapai デカパイ
kyonyuu 巨乳
bakunyuu 爆乳
Large breasts.
chippai チッパイ
hin'nyuu 貧乳
binyuu 微乳
Small breasts.
sentakuita
洗濯板
Washboard. (as in flat as a board.)
- sentaku - "laundry."
- The shape of a washboard resembles the ribs at the center of a flat chest.

Butt

denbu
臀部
Gluteal region. (technical.)
shiri 尻
oshiri お尻
Butt.
ketsu ケツ
ketsu 穴
ketsu 尻
Ass.
- ana 穴 = kun'yomi reading.
- ketsu 穴 = on'yomi reading.

koumon
肛門
Anus.
shiriana 尻穴
shiri no ana 尻の穴
ketsumedo 尻穴
"Butthole."
ketsu ana ケツ穴
ketsu no ana ケツの穴
"Asshole."
- In Gintama 銀魂, the name of the character Ketsuno Ana 結野アナ is a pun on the word for asshole in Japanese.
anusu
アヌス
"Anus."
anaru
アナル
"Anal." (e.g. sex.)
Anus. (the body part.)
- You read that right, in Japanese, "anal" also means "anus." Example:
- anaru wa iriguchi janai
  アナルは入り口じゃない
  ~~The anal isn't...~~ (wrong.)
  The anus isn't an entrance. (it's an exit.)

ari no towatari 蟻の門渡り
towatari 門渡り
Perineum.
"The entrance-traversing of ants."
- Because the perineum forms an ant-line-like line between one hole (genitals) and another (anus).
- to
  戸
  Door.
- wataru
  渡る
  To cross over. To traverse. (from one place to another.)

Penis

inkei
陰茎
Penis. (technical.)
penisu
ペニス
"Penis."
chinchin
ちんちん (or チンチン)
Penis. Wiener. (children's language.)
- ochinchin
  おちんちん (or おチンチン)
  Penis.^{(polite prefix.)}
- The word chinchin, without the o- prefix, also refers to a dog trick in which the dog puts its two front paws up plus a dozen other things.
chinko チンコ
ochinko おチンコ
Penis. Dick. (vulgar.)
chinpo チンポ
ochinpo おチンポ
Penis. Cock. (more vulgar.)
chinbou ちんぼう
chinpou ちんぽう
chinboko ちんぼこ
chinpoko ちんぽこ
chinpokko ちんぼっこ
Penis. (other variants.)

Character: Pes ペス
Anime: Kaguya-sama wa Kokurasetai ~Tensai-Tachi no Ren'ai Zunousen かぐや様は告らせたい～天才たちの恋愛頭脳戦～ (Episode 7, Cropped)

Context: making a dog sitting with its paws up is called a chinchin.

Also note this verb:

tsuku
付く
To attach. (literally)
- To have [a penis] attached.
- To have a penis.

Manga: Kaguya-sama wa Kokurasetai ~Tensai-Tachi no Ren'ai Zunousen~ かぐや様は告らせたい～天才たちの恋愛頭脳戦～ (Chapter 27, かぐや様は堪えたい)

ano, sono
あの　その
(stuttering.)
tsuiteru-n-desu ka?
ついてるんですか？
Is it attached?
- Do you have a penis?

Vagina

chitsu
膣
Vagina. (technical.)
nyoin (or join)
女陰
"Female genital." (some sources say this is the vulva, others that it's the vagina.)
manko マンコ
omanko おマンコ
Vagina. Pussy. (vulgar.)
- manko suru
  マンコする
  To have sex.
omeko
オメコ
Vagina. Pussy. (used more in Kansai.)
pusshii
プッシー
"Pussy."
- Also children's language for "cat."
- But wait, this is English, you knew that right?
wareme
割れ目 (or ワレメ)
Pudendal cleft. (slang.)
- wareru
  割れる
  To be parted, divided, split, cracked, etc.
- wareme-chan
  ワレメちゃん
  (cutesy way of saying.)
mekosuji
メコスジ
Camel toe.
- omeko no suji
  オメコのスジ
  The suji of an omeko.
- mansuji
  マンスジ
  (variant.)
- suji
  スジ
  A "thin line." (besides dozens of other meanings.)
  Camel toe. (because of the line between the labia.)
- The word mekosuji is sometimes abbreviated to just suji, and sometimes the adjective and noun switch places:
- sujimeko スジメコ
  sujiman スジマン
  Camel toe vagina.

Sometimes, manko is attached to words for other body parts used in sexual intercourse. For example:

ketsu-manko
ケツマンコ
Ass-pussy. (anal sex.)
- Originally a gay slang.
kuchi-manko
口マンコ
Mouth-pussy. (oral sex.)
nodo-manko
喉マンコ
Throat-pussy. (deep-throating.)
te-manko
手マンコ
Hand-pussy. (handjob.)
ashi-manko
足マンコ
Foot-pussy. (footjob.)

Censoring

Sometimes, manko, chinko and chinpo can get censored in manga by replacing a kana with a circle ◯. In anime, it gets bleeped instead.

Generally speaking, manko is more likely to be censored because it can also mean "to have sex," manko suru, so it ends up being sexual, while chinko is only genital.

For reference:

マ◯コ, マン◯, おマ◯コ, おマン◯
ま◯こ, まん◯, おま◯こ, おまん◯
manko, manko, omanko, omanko.
チ◯コ, おチ◯コ
ち◯こ, おち◯こ
chinko, ochinko.
◯ンポ, チ◯ポ, お◯ンポ, おチ◯ポ
◯んぽ, ち◯ぽ, お◯んぽ, おち◯ぽ
chinpo, chinpo, ochinpo, ochinpo.
チン◯, おチン◯
ちん◯, おちん◯
chinko, ochinko, or chinpo, ochinpo.
◯んこ, お◯んこ
◯ンコ, お◯ンコ
chinko, ochinko or manko, omanko.

Since chinchin, ochinchin is not considered vulgar—it's children's language—it's normally not censored, but for the sake of completeness:

◯んちん, ち◯ちん, ちん◯ん, ちんち◯
◯ンチン, チ◯チン, チン◯ン, チンチ◯
chinchin, chinchin, chinchin, chinchin
お◯んちん, おち◯ちん, おちん◯ん, おちんち◯
お◯ンチン, おチ◯チン, おチン◯ン, おチンチ◯
ochinchin, ochinchin, ochinchin, ochinchin

Other Terms

There are dozens of other colloquialisms, analogies and slangs for sexual body parts.

Let's start with the obvious ones for the testicles:

tama
玉
Balls. Ball. (literally.)
- Also a common name for a pet cat: tama タマ, for dog it's pochi ポチ, "spot."
kintama
金玉
Golden balls.
- In English, "jewels" are used similarly.
tamatama
タマタマ
Balls. ^{(reduplicated plural.)}
- Body terms featuring plurality by reduplication, otete お手手, omeme お目目, are normally used when talking to little children, babies, or in rare cases when someone is being condescending, treating someone else like a baby. In this case, it's because some adult women think tamatama is a cute-ish way of referring to the "balls" of an adult man.

For penis:

sao
竿
Stick. Pole.
nikusao
肉竿
Meat stick.
bou
棒
Stick. Pole.
nikubou
肉棒
Meat stick.

Some words that somehow also mean penis:

musuko 息子
gusoku 愚息
segare 倅
Son.
- Yes, "son," as in, not a daughter. Like "my son" is an innuendo for "my penis."
- itsumo musuko ga osewa ni natte-orimasu
  いつも息子がお世話になっております
  [My] son has been often in [your] care. (literally, a phrase you could use toward a teacher or babysitter who has taken care of your son.)
  [My] penis has been often in [your] care. (an internet meme comment posted in reply to pornographic content.)^{(nicovideo.jp:いつも息子がお世話になっております)}
ichimotsu
一物
"One thing."
- That's what it means. "One thing."
- Okay, it can also mean something you have "hidden," like in your chest, a secret, so maybe it comes from that meaning instead.
mono
モノ (or 物)
"Thing." (just "thing," like, "his thing," etc.)

Some words that don't mean penis, but get compared a lot:

kinoko
キノコ (or 茸)
Mushroom.
banana
バナナ
Banana.
sooseeji
ソーセージ
Sausage.
uin'naa
ウインナー
Wiener.
ottosei
オットセイ
Fur seal.
zou
象
Elephant. (because of the trunk.)
manmosu
マンモス
Mammoth.

Anime: Shimoneta to Iu Gainen ga Sonzai Shinai Taikutsu na Sekai 下ネタという概念が存在しない退屈な世界 (Episode 2)

A woman savoring a mushroom.

Manga: Kaguya-sama wa Kokurasetai ~Tensai-Tachi no Ren'ai Zunousen~ かぐや様は告らせたい～天才たちの恋愛頭脳戦～ (Chapter 27, かぐや様は堪えたい)

meimei chinko
命名　ちん子
Name: Chinko.
- ko 子
  A common suffix in names of girls. Yuuko 優子, Hanako 花子, etc.

There are even pairings like this mountainous one:

matsutake
松茸
Tricholoma matsutake, a mushroom.
(male genital.)
akebi
アケビ
Akebia quinata, the chocolate vine.
(female genital.)
[里のアケビは山のマツタケ見て開くってどういう意味ですか？ - detail.chiebukuro.yahoo.co.jp, accessed 2019-04-26]

Or this religious one:

mara
魔羅
In Buddhism, mara is a demon that tempted the Buddha with visions of seducing women.
(male genital.)
kan'non-sama
観音様
Avalokiteśvara, the bodhisattva. Guanyin.
(female genital.)

And of course, this militaristic one:

katana
刀
Sword. (penis)
saya
鞘
Scabbard. (vagina)
The term "vagina" comes from Latin, in which it means "sheath" or the "scabbard" of a sword. Similarly, "gladius," which means "sword," was a term for penis.^{(dartmouth.edu:Etymology of Abdominal Visceral Terms)}

Terms for the female genitalia:

awabi
鮑
Abalone. Ear shell. (from the shape of the vulva.)
mitsutsubo
蜜壷
"Honeypot." (vagina.)

Notably, specifically in regards to sex:

meiki
名器
Famed instrument/utensil. (an excellent vagina.)
- Often in the sense of:
- shimari ga ii
  締まりがいい
  "The contraction is good."
  Good at contracting.
- It can also mean "an excellent penis," apparently.

Specially because Japan has different weird terms for different types of "excellent vaginas," like:

mimizu senbiki
ミミズ千匹
One thousand worms. (from the texture of the vaginal walls.)
kazu-no-ko tenjou
数の子天井
Ceiling of herring roe. (from the texture of the superior vaginal wall in missionary position.)
- zarame
  ザラメ
  (synonym.)
- kazu no ko
  数の子
  Herring roe.
takotsubo
タコツボ (or 蛸壺)
Octopus pot. (a vagina which tightens on penetration.)
- Octopus pots are pots used to capture octopuses, by the way. They're placed horizontally on the sea floor, and octopuses get into them because they like to hide and there aren't many places to hide into on the sea floor.
kinchaku
巾着
Drawstring pouch. (a vagina that tightens entirely.)

Colloquialisms for the "clitoris," kuritorisu クリトリス, include:

kuri
クリ
Clit. Clitoris. (abbreviation)
- kuri
  栗
  Chestnut. (also means clitoris because it's a homonym.)
- kuri to risu
  栗と栗鼠
  Chestnut and squirrel. (only means clitoris if it's a pun.)
- kurichan クリちゃん
- maron マロン
  Marron. Culinary name for chestnut. (also slang for clitoris.)
kurikuri
クリクリ
(same meaning.)
- okurikuri
  オクリクリ
  (same meaning.)
kuritto
クリット
"Clit.
mame
豆
"Bean."
- omame お豆
  mamesan 豆さん
  omamesan お豆さん
  (same meaning.)
chikaku
恥核
"Shame core." (core as in the solid center of something, the pit of a fruit, etc.)
nyoshin
女芯
"Female core." (core as in the center of something.)
kashin
花芯
"Flower core." (i.e. pistils and stamens.)

Other terms about the female genitalia:

hanabira
花びら
Flower petals. (labia minora.)
- birabira
  ビラビラ
  (labia.)

poruchio
ポルチオ
"Portio" uteri. (ectocervix.)

Terms for the Mons pubis:

inpu
陰阜 (or inbu, infu)
Mons pubis.
deruta
デルタ
"Delta." Mon pubis.
- deruta chitai
  デルタ地帯
  "Delta zone." River delta.
- A river delta is a landform created by sediment carried by a river toward stagnant water (e.g. a lake, sea, ocean.)
- It's called "delta" because it's often shaped like a triangle, which's the shape of the Greek letter delta Δ.
- Colloquially, when a girl wearing a mini-skirt sits on a chair, with her knees facing to you, the space toward her crotch between her thighs and skirt which forms an inverted triangle may be called "delta" in Japanese.^{(wikipedia.org:デルタ)}
- And, similarly, the Mons pubis, which's on the crotch, is also called delta.
dote
土手
"Embankment."
- A bank of earth built to stop water, wind.
- Since this "bank" is an elevated mound of earth, it ends up referring to the pubic mound too.
- doteman
  ドテマン
  (sex-related meaning only.)
moriman
モリマン
From moriagaru 盛り上がる, "to swell," "to bulge."

Other slangs for body parts:

In Japanese, bii-chiku Ｂ地区, literally "district B," is a slang for chikubi 乳首, "nipple."

Anime: Shimoneta to Iu Gainen ga Sonzai Shinai Taikutsu na Sekai 下ネタという概念が存在しない退屈な世界 (Episode 1)

biichiku
Ｂ地区
"District B."
Nipple. (because it's chikubi.)

With adjectives:

kyokon 巨根
dekachin デカチン
dekamara デカマラ
kyochin 巨チン
Large penis.
- dekai
  デカイ
  Big.
tanshou
短小
"Short and small."
- mijikai
  短い
  Short.
- chiisai
  小さい
  Small.
sochin
粗チン
Small penis.
- somatsu
  粗末
  Poor. Shabby.
yuruman
ユルマン
Loose vagina.
- yurui
  緩い
  Loose.
kitsuman
キツマン
Tight vagina.
- kitsui
  きつい
  Tight.

Other related terms

paipan
パイパン
Having no pubic hair. (usually female.)
- This term comes from Mahjong. (yes, really.)
- In Japanese, the white dragon tile is called paipan 白板, pai means white, pan means board, so "whiteboard." That's not the normal reading of the kanji. The term is probably loaned from Chinese.
chinge
ちん毛
(male) pubic hair. "Penis hair."
mange
マン毛
(female) pubic hair. "Vagina hair."
kemeko
毛めこ
Hairy vagina.
houkei
包茎
Phimosis.
kyosei
去勢
Castration.
- Similar words:
- kyousei
  共生
  Living together.
- kyousei-teki ni
  強制的に
  Forcibly.

Menstruation

Menstruation, or period, is a thing that occurs periodically, like, per month, which in Latin is "mensis," which is also more or less how periodically the Moon goes around the Earth.

gekkei
月経
Menstruation. (technical.)
- getsu
  月
  Month.
- tsuki
  月
  Moon.
- tatsu
  経つ
  (for time) to pass.
- gekkei-zen shoukougun
  月経前症候群
  "Premenstrual Syndrome." PMS.
shochou 初潮
shokei 初経
First menstruation. Menarche.

Colloquially, it's also called:

seiri
生理
Physiology.
Menstruation.
- seiri-tsuu
  生理痛
  Menstrual pain.
- seiri-kikan
  生理期間
  Menstrual interval.
- seiribi
  生理日
  Menstrual days.
- seiri genshou
  生理現象
  Physiological phenomenon.
mensu
メンス
"Mens." (German word for menstruation.)
onna no ko no hi
女の子の日
Girl's day. Girl's days.

It's also called in some particularly ambiguous ways:

ano hi
あの日
That day. Those days.
are
アレ
That.

As for the products for menstruation:

tanpon
タンポン
Tampon.
- apurikeetaa
  アプリケーター
  Applicator.
napukin
ナプキン
"Napkin." (yes, just napkin.)
Pad. Menstrual pad. Sanitary pad. Sanitary towel. Sanitary napkin.
gekkei kappu
月経カップ
Menstrual cup.

Anime: Oshiete! Gyaruko-chan おしえて! ギャル子ちゃん (Episode 2)

napukin-ha wa shojo,
tanpon-ha wa hishojo tte
hontou desu ka?
ナプキン派は処女、
タンポン派は非処女って
本当ですか？
Is it true that
[those that prefer] pads are virgins,
[those that prefer] tampons are non-virgins?
(contrastive wa は.)
- ~ha
  ～派
  Having preference for [something]. (suffix.)

Erection

The word for erection in Japanese is:

bokki
勃起
Erection.
- Normally refers to the penis, but can also be clitoris, nipples.
- Doesn't mean the "erection" a building.
bokki suru
勃起する
To get erect.
tatsu
立つ
To stand up. (general term.)
- Sometimes spelled with ateji from the kanji of bokki 勃起.
- tatsu
  勃つ
  To stand erected.
  (for a penis) to be erect.
- tatsu
  起つ
  To rise up. (politically.)
  To get erect. (sexually.)

Related terms include:

tento wo haru
テントを張る
To put up a tent. (like in camping.)
- Also to have an erection while wearing pants, because of the tent-like shape it makes.
chin-piku
チンピク
"Penis-twitch."
- A portmanteau of chinchin with pikupiku ピクピク, a mimetic word for "twitching."
yakan inkei bokki genshou
夜間陰茎勃起現象
"Nocturnal Penis Erection Phenomenon."
Nocturnal Penile Tumescence. NPT. (technical term.)
- Also known as:
- asadachi
  朝立ち (or 朝勃ち, 朝起ち)
  Morning wood.
- yorudachi
  夜立ち (夜勃ち, 夜起ち)
  (if it happens at night.)
bokki fuzen
勃起不全
"Incomplete erection."
Erectile dysfunction.
- ee-dhii
  イーディー
  ED.
- bokki kinou shougai 勃起機能障害
  bokki shougai 勃起障害
  "Erection function impediment."
- in'i
  陰萎
  Erectile impotence.
- inpotentsu
  インポテンツ
  "Impotent."
- inpo
  インポ
  (abbreviation, sometimes used derogatorily.)
- inpo yarou!
  インポ野郎！
  You impotent bastard! (or something like that.)

Fluids

ekitai
液体
Fluid.
taieki
体液
Bodily fluid.
nen'eki
粘液
Mucus.

seieki
性液
Sexual fluid. (not a technical term, colloquialism.)

chi
血
Blood.
- chi ga deru
  血が出る
  Blood leaves.
  To bleed.
- shukketsu
  出血
  Bleeding.
- ketsueki
  血液
  Blood. (anatomical sense.)
bonyuu
母乳
Maternal milk.
- bonyuu ga deru
  母乳が出る
  (same idea.)
ase
汗
Sweat.

chikou
恥垢
Smegma. (technical.)
chinkasu
チンカス
"Penis waste."
- Sometimes used as a derogatory:
- chikasu yarou!
  チンカス野郎！
  You smegma bastard! (what? This makes no sense in English.)
kurikasu
クリカス
"Clitoris waste."
mankasu
マンカス
"Vagina waste."

toumei
透明
Clear. Transparent.
usui
薄い
Thin. Watery.
koi
濃い
Thick. Dense.
nigai
苦い
Bitter.
shoppai
塩っぱい
Salty.

Pee

nyou
尿
Urine. (technical.)
shouben
小便
Pee.
- daiben
  大便
  Poop.
- In a way, the naming is similar to "number 1" and "number 2" in English, except it's "small" (小) and "big" (大) in Japanese.
oshikko おしっこ
shikko しっこ
Pee. Piss.
- In English there's a number of idioms involving the word "piss," like "piss off," "to be pissed," and so on that absolutely don't translate to the words for piss in Japanese.

hainyou
排尿
Urination. "Urine expelling."
hounyou
放尿
Urination. "Urine release."
hainyou suru 排尿する
hounyou suru 放尿する
To urinate.
oshikko suru
おしっこする
To pee.

shikkin
失禁
Incontinence. (technical.)
omorashi
お漏らし
Incontinence. Wetting one's pants. (children's language)
- morasu
  漏らす
  To leak. (transitive.)

Semen

Semen is a liquid composed by sperm and other stuff.

seieki 精液
seisui 精水
Semen.
- seieki saishu
  精液採取
  Semen collection.
seishi
精子
Sperm.
- seishi banku
  精子バンク
  Sperm bank.
seishou
精漿
Everything that's not sperm in the semen.
- zenritsusen-eki
  前立腺液
  Prostatic fluid.
- seinou-bunpitsu-eki
  精嚢分泌液
  Seminal vesicle secretion.
- nyoudou-kyuusen-eki 尿道球腺液
  kaupaasen'eki カウパー腺液
  Pre-ejaculate. Cowper's fluid. "Cowper's gland fluid."

Note that a lot of people don't know the difference between the two and they will mistake "semen" for "sperm" and vice-versa, both in English and in Japanese.

zaamen
ザーメン
Cum. "Semen."
- A katakanization, but likely associated with pornography.^{(chiebukuro.yahoo.co.jp:ザーメンってなんですか？)}
zaajiru
ザー汁
"Semen juice." (a lot of slangs have jiru 汁 in it.)

Slangs for "pre-cum," pre-ejaculate, include:

kaupaa
カウパー
"Cowper." (fluid.)
gaman-jiru
我慢汁
"Endurance juice." "Patience-juice."
- gaman suru
  我慢する
  To endure. To have patience.

Semen is sometimes likened to white drinks, like:

miruku
ミルク
Milk.
yooguruto
ヨーグルト
Yogurt.
kefia
ケフィア
Kefir.
karupisu
カルピス
Calpis. (Japanese soft drink.)

By its smell:

ika-kusai
イカ臭い
Squid smelling.
- This analogy was brought to you by a country that eats a lot of seafood.

Vaginal Discharge

Vaginal discharge is also composed by a bunch of things.

chitsu bunpitsueki
膣分泌液
Vaginal discharge.
- chitsu-eki
  膣液
  Vaginal fluid.
sukiinsen-eki
スキーン腺液
Skene's gland secretion.
barutorinsen-eki
バルトリン腺液
Bartholin's gland secretion.
shikyuukeikan nen'eki
子宮頚管粘液
Cervical mucus. "Cervical canal mucus."

The amount of fluid discharged increases while sexually aroused.

nureru
濡れる
To get wet. (intransitive.)
- nurasu
  濡らす
  To wet. (transitive.)
- In anime, there are some series with female characters whose archetypal personality is basically "horny," who will cling to their romantic interest or with everyone basically, who will exclaim nureru to say "you'll make me wet" as a joke (in the sense it's part of the humor of the series, the character, however, is totally serious about it). Even trap characters may say this as a joke.
- If I remember correctly, Cthuko, クー子, from Haiyore! Nyaruko-san 這いよれ！ニャル子さん may have done this, for example.
- This is worth of note because I don't think there's a male equivalent. Male characters don't say something like "you'll make me hard" in the same horny joke sense as this. In particular, Nyaruko-san isn't really about shimoneta or even overly erotic, it's mostly comedy, so you can have a series that looks mostly innocent, but will have a sentence such as this, because it's actually a moe series targeted at otaku, so it assumes its audience won't be fazed by this sort of horny.

There's a term specifically for vaginal discharge while not aroused:

orimono
オリモノ (or 下り物)
"Falling thing." (normal vaginal discharge.)
- Synonyms:
- koshike こしけ
- taige 帯下

Slangs include:

aieki
愛液
"Love fluid."
manjiru
マン汁
"Pussy juice."
rabu juusu
ラブジュース
"Love juice." (may also mean semen.)

Masturbation

jii 自慰
jidoku 自涜
Masturbation.
jibun wo nagusameru
自分を慰める
To console oneself. (literally.)
To masturbate. (idiom.)
masutaabeeshon マスターベーション
masu マス
"Masturbation." (katakanization.)
hitori-ecchi
一人エッチ
Sex alone.
senzuri
千摺り
(male) masturbation.
- suru
  摺る
  To rub. (suri, "rubbing," -zuri ～ズリ is rendaku.)
- So it means "to rub one out," basically.
- manzuri
  マンズリ
  (female) masturbation.
- paizuri
  パイズリ
  "Boob-rubbing." Boob-job.
- chikubizuri
  乳首ズリ
  "Nipple-rubbing."
onanii
オナニー
Masturbation. (from Onan, son of Judah, from the Bible.)
- onanii suru オナニーする
  onaru オナる
  To masturbate.
- onanizumu
  オナニズム
  Onanism. Masturbation.
  Indulging in self-pleasure instead of doing something properly. (e.g. in sports, arts, etc.)
- onakin
  オナ禁
  Masturbation "ban." Abstaining from masturbating for some reason for a period of time.
- onahooru オナホール
  onaho オナホ
  "Onahole." A kind of sex toy in the shape of a hole.
- onabare
  オナバレ
  To be caught masturbating.
- kuriona
  クリオナ
  Clitoral masturbation.
- chikunii
  チクニー
  Nipple masturbation.
- ananii
  アナニー
  Anal masturbation.
- kado-ona
  角オナ
  "Corner masturbation." (in the sense of rubbing your genitals against the corner of a desk or table.)
- yuka-ona
  床オナ
  "Floor masturbation." (in the sense of dry humping the floor.)
shuin
手淫
Masturbation. (using hands.)
- This term can also mean masturbating someone else.
jika hatsuden
自家発電
Generating electricity at home. (e.g. with solar panels.)
Masturbation.
shigoku 扱く
koku 扱く
To stroke.
tekoki
手コキ (or 手扱き)
(male) hand-job. "Hand-stroking."
- teman
  手マン
  (female) fingering.
- ashikoki
  足コキ
  Foot-job. "Foot stroking."
- shirikoki
  尻コキ
  "Butt stroking." (stimulated by one's buttocks without penetration.)
- wakikoki
  腋コキ
  "Armpit stroking." (you get the idea.)
ijiru
弄る
To touch with one's hands, changing or probing it somehow. To fiddle with.
- E.g. to fiddle (ijiru) a machine to figure out how it works, to fiddle with one's hair, to play with one's nipples while masturbating, to change the settings of a NPC character your friend made and end up making her fall in love with you somehow, etc.
- sawaru
  触る
  To touch. (deliberately.)
- furu
  触る
  To touch. (when two things happen to touch, whether deliberately or not.)
- kosuru
  擦る
  To rub. To touch against. (e.g. to rub against the fabric of one's clothes.)
shikoru
シコる
To fap.
- shikoshiko
  シコシコ
  *fap fap.*
  (mimetic word.)

And a related word:

thisshu
ティッシュ
Tissue.
- In anime, sometimes a tissue box is shown randomly, e.g. while the character talks about something else, and nothing is noted about the box. This can be a (not really) subtle way to hint what the character has done.

つまり･･･だから　その･･･ママで始まる言葉で･･････マ･･･マス･･････その　つまりマスタ～～･････････～～ベション･･･････････････をよ･･････

Manga: JoJo no Kimyou na Bouken ジョジョの奇妙な冒険 - Part 5: Stone Ocean (Chapter 1, 石作りの海（ストーンオーシャン）　その①)

Context: a girl tells what she was seen doing.
tsumari... dakara
sono... ma
つまり･･･だから　その･･･マ
(stuttering) ma
ma de hajimaru kotoba de
マで始まる言葉で
A word that starts with "ma."
...... ma...
･･････マ･･･
...... ma...
masu......
マス･･････
Mas...
sono, tsumari
その　つまり
(more stuttering)
masuta~~
マスタ～～
Mastur~~
.........~~~~~beshon...............
･････････～～ベション････････････
......~~~~bation............
... wo yo......
･･･をよ･･････
(incomplete sentence ending in object marker: what was said previous was what she was seen doing.)

オカズ

The word okazu オカズ normally means "side dish" in Japanese, as in, food, but it can also means "something your masturbate to." For example: just plain old pornography, naked selfies from your girlfriend, a lemon fanfic of Dobby x Lucius Malfoy, wishful fantasies of getting laid with a hot celebrity from one of the Avengers movies, the paint drying on the wall, or whatever it is you saw and declared "I can fap to this."

__ wo okazu ni shita
〇〇をオカズにした
To have made ____ into [one's] "okazu."

Similar terms include:

zurineta
ズリネタ
"Rubbing content."
- The zuri is from senzuri, manzuri, and neta means "content," like the content of an article, or the plot of a story.
shikorithi
シコリティ
"Shiquality." (from shiko and "quality.")
- How good a zurineta is.
- shikorithi ga takai
  シコリティが高い
  Shiquality is high. (i.e. something good to masturbate to.)
onapetto
オナペット
"Masturbation pet." "Onapet."
- From onanii and "pet," petto.
- A person repeatedly used as zurineta. (or a video, etc.)
- onadoru
  オナドル
  A person who became an "idol," aidoru アイドル, as in, a type of celebrity, in order to become an onapet for their fans.

Ejaculation

shasei
射精
Ejaculation. (technical.)
- iru
  射る
  To shoot. (e.g. an arrow.)
- gansha
  顔射
  Facial. "Face shot." (ejaculating on someone's face.)
shasei suru
射精する
To ejaculate.
- shasei suru
  写生する
  (totally not sexual homonym.)
  To sketch something while looking at it, as opposed to from your imagination.
hassha
発射
Firing. Catapulting.
Ejaculation. (slang.)
- In anime with giant robots, space rockets, etc. when they are put on those "launching platforms," hasshadai 発射台, this is the word the operator says before "launching" them.
seitsuu
精通
First ejaculation.
- shosei
  初精
  The first semen ejaculated. (note: in English spermarche, semenarche, is the "start of sperm development.")
musei
夢精
Nocturnal emission. Wet dream.
- yume
  夢
  Dream.
- isei
  遺精
  Ejaculation without sexual stimulation. (musei is one type of isei.)
sourou
早漏
Premature ejaculation.
- hayai
  早い
  Early.
- moru
  漏る
  To leak. (intransitive.)
shiofuki
潮吹き
Female ejaculation. Squirting.
"Sea water blowing."
- From whales blowing water upwards with their blowholes.
- shio
  潮
  Sea water.
- fuku
  吹く
  To blow.

A bunch of verbs that should have nothing to do with it, but do anyway:

deru
出る
To leave. To flow out. (e.g. ejaculate)
dasu
出す
To make leave. To put out. (i.e. to ejaculate.)
To take out. (e.g. to take your penis out of your pants.)
- nakadashi
  中出し
  "To put out" inside. (cumming inside.)
- sotodashi
  外出し
  "To put out" outside.
nuku
抜く
To extract. (e.g. ejaculate.)
- Sometimes it means "to masturbate."
- nukigee
  抜きゲー
  "Extracting game." A kind of adult video game.
tamaru
溜まる
To accumulate.
- In the sense of not ejaculating as often as one would like.
- Many dirty jokes involve this word because it can normally refer to stress accumulating, or work piling up, etc.
kakeru
かける
To pour onto.
- bukkake
  ぶっかけ
  Pouring onto.
- bu'~
  ぶっ～
  Prefix that implies some force or impact.
- bukkorosu
  ブッ殺す
  I WILL MURDER YOU. (korosu 殺す, "kill.")
- buttobasu
  ぶっ飛ばす
  To send someone flying, like with a punch. From tobasu 飛ばす, "to make fly."
shiboru
搾る
To squeeze out [fruit juice].
- sakusei
  搾精
  Semen milking.

Sex

seikou
性交
Sexual intercourse.
seikoui
性行為
Sexual act.

セックス

The most common colloquialism for sex would be:

sekkusu
セックス
"Sex."
sekkusu suru
セックスする
"To sex." To have sex.

Some related terms:

sekuhara
セクハラ
Sexual harassment.
- sekushuaru harasumento
  セクシュアルハラスメント
  (same meaning.)
- See also: OL (Office Lady).
sekukyaba
セクキャバ
Sex-cabaret.
- A cabaret club, kyabakura キャバクラ, in which the employees are topless or wearing sexy outfits.
sefure
セフレ
"Sex friend." Fuck buddy. (a friend whom you have sex with.)
- sekkusu furendo
  セックスフレンド
  (same meaning.)
kimeseku
キメセク
Sex on drugs. (rock and roll not a requisite.)
- kimeru
  キメる
  To take [a drug]. (transitive.)
  To strike a pose. (e.g. making a peace sign.)
  To score a goal.
  To finish well something.
- kimaru
  キマる
  To get high. (intransitive.)

Some people avoid saying the word sekkusu because it sounds vulgar, preferring something like:

ecchi
エッチ
Sex.
Sexy.
Perverted.
- Also spelled as an "H."
- In fact it comes from the letter H, because it comes from the romaji of hentai 変態, which means "pervert."
ecchi suru
エッチする
To have sex.
- hitori-ecchi
  一人エッチ
  Sex alone. Masturbation.
neru
寝る
To sleep.
To sleep [with someone].
- Also where comes the infamous terms:
- netoru
  寝取る
  To take "sleeping."
  To sleep [with someone else's partner].
- netorareru
  寝取られる
  (passive form.)
  To be netoru'd
  For someone who's not your partner to sleep with you.
- netorare
  寝取られ
  (the act of the above.)
  Cheating. Cuckolding. (genre.)
- netorase
  寝取らせ
  Letting netoru. To let someone else sleep with your partner.
  - Generally speaking, netorare as a genre is common in fan art because it uses a character from a series with already-established relationships. If any heroine has sex with anyone but the main character whom they canonically like, that can be netorare, and since they like that character, they wouldn't cheat on him willingly, which results in the netorare genre being infamously associated with non-consensual scenarios. By contrast, netorase means there's consent of the party being cheated on, so it's closer to fetishistic cuckolding in real life.
suru
する
To do.
- Just... "to do," e.g. in the volitional form shiyou しよう, "let's do [it]," meaning sekkusu shiyou, "let's have sex." This isn't a separate meaning of the verb suru, this is the suru-verb sekkusu suru or something alike with the verbal noun sekkusu omitted as it's inferred pragmatically.

Or something so far-fetched nobody will actually understand it:

nakayoshi
仲良し
To make peace. "To get on good terms."

Or something that people don't really say but you may see in anime:

gattai
合体
To combine.
- Like giant robots!

Anime: Seitokai Yakuindomo 生徒会役員共 (Episode 8)

Context: gattai!

Then we have this extremely confusing word:

yaru
やる
- To do something.
- Synonymous with ageru:
  - To give something to someone.
  - To do something for someone.
- To kill someone.
- To have sex with someone

And yes, it actually means all that stuff:

yaritai koto
やりたいこと
Something [you] want to do.
banana wo yaru
バナナをやる
[I'll] give [you] a banana.
oshiete-yaru
教えてやる
[I'll] teach [you]. [I'll] tell [you].
kare wo yatta zo
彼をやったぞ
[I] killed him.

そう思っていると突然その男は僕の見ている目の前でツナギのホックをはずしはじめたのだ･･･！やらないか

Manga: Kuso Miso Technique, くそみそテクニック

sou omotte-iru to
totsuzen sono otoko wa
{boku no mite-iru} me no mae de
tsunagi no hokku wo
hazushi-hajimeta no da...!
そう思っていると突然その男は僕の見ている目の前でツナギのホックをはずしはじめたのだ･･･！
While [I] was thinking like that, that man, in front of [my] eyes [while] {I was seeing [him]}, started undoing the hook of [his] jumpsuit.
yaranaika
やらないか
Won't [you] do [it]? (do it = have sex.)

It's seen in the words:

yari-nige
やり逃げ
To do and run away.
To have sex with someone and then "run away," i.e. never contact them again.
yari-chin
ヤリチン
A promiscuous man.
- A doing (yari) penis (tin), in the sense it does it a lot.
yari-man
ヤリマン
A promiscuous woman.

Similar words:

bicchi
ビッチ
Slut. (wasei-eigo, from the English "bitch.")

Fujiwara Chika 藤原千花, visual pun on the word shirigaru-onna 尻軽女, "light-butt-woman," meaning a promiscuous woman, using balloons spelling the English word "slut."

Character: Fujiwara Chika 藤原千花
Anime: Kaguya-sama wa Kokurasetai? ~Tensai-Tachi no Ren'ai Zunousen~ かぐや様は告らせたい？～天才たちの恋愛頭脳戦～ (Season 2) (Episode 3)

shiri-garu-onna
尻軽女
A promiscuous woman.
- shiri ga karui
  尻が軽い
  [Her] butt is light. (literally.)
  [She] is promiscuous. [She] is a slut.

Other terms:

omanko suru おまんこする
omeko suru おめこする
To have sex.
- From the slangs for vagina: omanko, omeko.
- So "to vagina," literally.
hameru
ハメる (or はめる)
To put in. (e.g. your hand into a glove, or wrist into handcuffs.)
To have sex. (colloquialism.)
- hame-dori
  ハメ撮り
  "Sex [film] taking."
  POV pornography. A sex video in which someone who's having sex films it, as opposed to a third party.
- toru
  撮る
  To take [a photo]. To take [a film].
horu
ほる
To dig.
To have gay sex. (colloquialism. See okama オカマ.)

Terms related to real-life gay relationships:

neko
ネコ
Bottom.
tachi
タチ
Top.
riba
リバ
"Reversible." Switch.
凸凹回
Top, bottom, switch. (symbols used in dating sites.)

Terms used mostly by fujoshi characters shipping fantasy gay relationships:

seme
攻め
"Offense." Attack.
Top.
Assertive.
uke
受け
"Receive."
Bottom.
Passive.
mamori
守り
"Defense."
- Not a sexual slang, but you may hear a fujoshi character ask "what's the opposite of seme (offense/top)?" A normal character will answer mamori (defense), a savvy character will answer uke (bottom).

(wait a second...???)

seiteki shigeki
性的刺激
Sexual stimulation.
seiteki koufun
性的興奮
Sexual excitement. Arousal.
- koufun suru
  興奮する
  To get excited. (not necessarily sexual.)
seikantai
性感帯
Erogenous zone. "Sexual feeling region."
yokubou
欲望
Desire. (generic sense.)
- jouyoku
  情欲
  Lust.
- seiyoku
  性欲
  Sexual desire.
uzuku
疼く
To throb painfully.
hatsujou
発情
Desire to mate.
- hatsujouki
  発情期
  Mating season. (of animals.)

There are various ways to say "sexy," like:

eroi
エロい
Erotic.
- ero-kawaii
  エロ可愛い
  Sexy and cute.
ecchi na
エッチな
Lewd.
iroppoi
色っぽい
Sexy.
- A combination of two words that certainly shouldn't mean sexy but do.
- iro
  色
  Color.
- ~ppoi
  ～っぽい
  Like. Resembling.
- inu ppoi
  犬っぽい
  It's kind of like a dog.
  It resembles a dog.
sekushii
セクシー
"Sexy."

Verbs related to sexual intercourse:

sounyuu suru
挿入する
To introduce.
To penetrate.
ireru
入れる
To put in. (general.)
- ireru
  挿れる
  To put in. (sexual.)
tsuku
突く
To thrust.
ugokasu
動かす
To make [something] move. (transitive.)
- ugoku
  動く
  To move. (intransitive.)
pisuton
ピストン
Piston. (as in the pistoning movement.)
shimaru
締まる
To tighten.
To contract.
shime-tsukeru
締め付ける
To tighten and adhere to.

Other related words:

zengi
前戯
Foreplay. (stuff before sex.)
honban
本番
Actual performance. (in TV shows, when it's not a rehearsal, test, etc. it's for real.)
Sex. Vaginal penetration.
- Sometimes associated with prostitution, because a couple won't call "sex" honban, it's only honban in contrast to whatever part of it isn't honban.
kougi
後戯
(stuff after sex.)
nama
生
Raw. (as opposed to protected sex.)
- nama-hame
  生ハメ
  Raw sex. Bareback.
adaruto bideo
アダルトビデオ
"Adult video."
- ee-bui
  エーブイ
  AV.

Sex Positions

taii
体位
Position of the body.
Sexual position.
seijoui
正常位
"Normal position."
Missionary.
kijoui
騎乗位
"[Horse] Mounting Position."
Cowgirl.
- gyaku-kijoui
  逆騎乗位
  Reverse cowgirl.
  (in the cowgirl position the girl mounts the guy facing him, while in the reverse cowgirl position they both face the same direction so the girl has her back toward him.)
kouhaii 後背位
bakku バック
"Behind back." "Back."
Doggy style.
zai
座位
"Sitting position."
Lotus position.
sokui
側位
"Lateral position."
Spooning. Spoons position.
shikkusu-nain
シックスナイン
"Six nine." Sixty nine. 69.
paizuri
パイズリ
Boobjob.
- gyaku-paizuri
  逆パイズリ
  Reverse boobjob. (literally.)
  (a paizuri from behind.)

Sex Toys

There are various terms for sex toys, and tools, in Japanese:

seigu
性具
"Sexual tools."
ingu
淫具
"Lewd tool."
otona no omocha
大人のおもちゃ
Adult toy.
- Note that omocha is also spelled omocha 玩, from:
- moteasobu 玩ぶ
  To play with [something you hold in hands].
  To toy with [somebody's emotions].

Some terms for toys and tools:

dhirudo
ディルド
Dildo.
- harikata
  張形 (or harigata)
  "Swollen form." Dildo. (note: the Hector's dolphin is called seppari iruka 背張海豚, due to its "swollen back.")
- kokeshi
  コケシ
  Dildo. (which's shaped like a kokeshi doll.)
baibu バイブ
baibureetaa バイブレーター
"Vibrator."
- shindou
  振動
  Vibration.
- pinku rootaa ピンクローター
  rootaa ローター
  "Pink rotor." Love egg. Egg vibrator.
- rimokon
  リモコン
  Remote control.
Hitachi majikku wando
日立マジックワンド
Hitachi Magic Wand.
anaru puragu
アナルプラグ
Anal plug. Butt plug.
anaru biizu
アナルビーズ
Anal beads.
kanchou
浣腸
Enema.
onahooru オナホール
onaho オナホ
Onahole. Sex sleeve.
dacchi waifu
ダッチワイフ
"Dutch wife." Sex doll. Blowup doll. (not to be confused with trash.)
- rabu dooru
  ラブドール
  "Love doll." (refers to a high-quality type.)
enemagura
エネマグラ
Prostate massager.
"Enemagra." (proprietary eponym.)
- zenritsusen massaaji-ki
  前立腺マッサージ器
  Prostate massage utensil. (generic name.)
- anerosu
  アネロス
  Aneros. (another brand.)
peniban ペニバン
penisubando ペニスバンド
"Penis-band." Strap-on.

Character: Kuroki Tomoko 黒木智子
Anime: Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui! 私がモテないのはどう考えてもお前らが悪い！ (Episode 4)

Context: a Hitachi wand, which is marketed as an electric body massager, used like this, to massage yourself.

Orgasm

zecchou
絶頂
"Climax."
- This can be the climax of a story, the peak of a mountain, etc.
- seiteki zecchou
  性的絶頂
  Sexual climax. (unambiguous.)
oogazumu
オーガズム
"Orgasm." (English.)
orugazumu
オルガスムス
"Orgasm." (German.)
akume
アクメ
Orgasm. "Acmé" (French.)
dorai oogazumu
ドライオーガズム
"Dry orgasm."
tokoroten
ところてん (or 心太, 心天, 瓊脂)
A dish made from tengusa 天草, a type of red algae, using a special tool called ten-tsuki 天突き, "heaven thrusting," (but the "heaven," ten, probably refers to tengusa, tokoroten), with which the tengusa, after turned into jelly, is pushed into a cheese-grater kinda thing with holes and comes out as noodles.
- A video, if you really want to know: ところてん（テングサ）の作り方 - youtube.com.
- From the way it's made:
- tokoroten-shiki
  ところてん式
  Tokoroten ceremony.
  When someone is naturally pushed out, usually in a bad sense. For example, in Sumo wrestling there was a rule that there could only be five oozeki 大関. But then a sixth one showed up. So the oldest one of the five was "pushed out" to the next rank, yokodzuna 横綱, in order to maintain five oozeki.
- And finally, what's relevant to this article:
- tokoroten
  ところてん
  For a man to have an orgasm from anal alone. (in the sense of "pushing out" the semen from behind.)

In English, the verb "to cum" is a slang for "to orgasm." It's sometimes spelled "to come." Such that "I'm coming" becomes a double entendre. In Japanese, the word for "to come" is kuru 来る, but it doesn't carry this orgasm meaning.

On the other hand, the word iku 行く, "to go," does mean "to orgasm" in Japanese. So "to come" is English and "to go" is Japanese. Why, though? Is this some sort of coincidence or something? Was this on purpose? Anyway, it's normally spelled with katakana, not with kanji:

iku
イく (or イク)
To cum. To orgasm.
- When conjugated, it's homonymous with other words, like:
- iu
  言う
  To say.
- itta
  言った
  [I] said.
- itta
  イった
  [I] came.

Terms for orgasm expressions:

ikigao
イキ顔
"Cumming face." (from iku.)

Araragi Karen 阿良々木火憐, example of torogao トロ顔. Picture from the infamous toothbrush scene.

Character: Araragi Karen 阿良々木火憐
Anime: Nisemonogatari 偽物語 (Episode 8)

Context: the teeth-brushing scene.
torogao
トロ顔
"Dozy face" (from torotoro トロトロ, "dozing off.")

Kuroki Tomoko 黒木智子, example of ahegao アヘ顔. This image was a single frame Easter egg hidden in the ED of the 5th episode.

Character: Kuroki Tomoko 黒木智子
Anime: Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui! 私がモテないのはどう考えてもお前らが悪い！ (Episode 5)

Context: an Easter egg hidden in the outro of the 5th episode.
ahegao
アヘ顔
"Panting face." (from aheahe アヘアヘ, *pant pant*)
ahegao daburu piisu
アヘ顔ダブルピース
Ahegao double peace [sign]. Abbreviated AWP. In Japan, there's a culture of making peace signs when taking photos in general, and it also occurs when taking sexual photos (hame-dori), hence this the existence of this term.

特にＺＺ（ツインゼット）と弐号機が同時に絶頂を迎えるときの表情がもう最高ですねこんないいアヘ顔を描ける作家はそうそういませんよ

Manga: Boku wa Tomodachi ga Sukunai 僕は友達が少ない

toku ni {ZZ (tsuinzetto) to nigouki ga
douji ni zecchou wo
mukaeru} toki no hyoujou ga
mou saikou desu ne
特にＺＺ（ツインゼット）と弐号機が同時に絶頂を迎えるときの表情がもう最高ですね
In particular, the expression [on their faces] when {Twin-Z and Machine #2 simultaneously approach climax} is the greatest.
konna ii ahegao wo kakeru
sakka wa sousou imasen yo
こんないいアヘ顔を描ける作家はそうそういませんよ
There aren't so many artists who can draw ahegao this good.

Virginity

In Japanese, there are two different terms for "virgin."

doutei
童貞
Virgin. (usually male.)
shojo
処女
Virgin. (female.)

About one's first time:

shotaiken
初体験
First experience.
- hajimete
  初めて
  First time.
doutei soushitsu 童貞喪失
shojo soushitsu 処女喪失
Losing [one's] virginity.
sotsugyou suru
卒業する
To graduate. (literally.)
- daigaku wo sotsugyou suru
  大学を卒業する
  To graduate university.
- doutei wo sotsugyou suru
  童貞を卒業する
  To graduate [being] a virgin.
fude-oroshi
筆おろし
Lowering a brush.
- In the sense of lowering it onto a canvas below, to use it, for the first time.
- For a guy to lose his virginity.
- orosu
  下ろす
  To lower. To put down.

Regarding romantic assertiveness:

soushokukei-danshi
草食系男子
Herbivorous guys.
- Men who aren't romantically assertive, who don't go after girls.
- nikushokukei-joshi
  肉食系女子
  Carnivorous girls. (romantically assertive women.)

Conception

ninshin
妊娠
Conception. Pregnancy.
haramu
孕む (or 妊む)
To become pregnant with [a child].
- ko wo haranda
  子を孕んだ
  To have become pregnant with a child.
- haramaseru
  孕ませる
  To make [someone] become pregnant with [a child]. (causative conjugation.)
  To impregnate.
umu
生む
To give birth to [a child].
- ko wo unda
  子を生んだ
  To have given birth to [a child].
tainai
胎内
Interior of the womb of a pregnant woman.
kodzukuri
子作り
"Baby-making." Having children. Attempting to conceive.
hinin
避妊
"Avoid pregnancy."
Contraception. Birth control.
- sakeru
  避ける
  To avoid.

Notable contraception methods:

kondoomu
コンドーム
"Condom."
- Also called:
- gomu
  ゴム
  "Rubber."
- sukin
  スキン
  "Skin."
- furenchi retaa
  フレンチレター
  "French letter."
gomu-ari
ゴムあり
Condom existing.
With a condom.
- gomu-nashi
  ゴムなし
  Condom non-existent.
  Without a condom.
keikou hinin yaku
経口避妊薬
"Oral contraceptive medicine."
Birth control pill.
- piru
  ピル
  The "pill."

An antiquated device that's part of some jokes from time to time:

teisoutai
貞操帯
Chastity belt.
- teisou wo mamoru
  貞操を守る
  To protect [one's] chastity.

フェラチオ

Regarding specifically oral sex:

ferachio フェラチオ
fera フェラ
Fellatio. Blowjob.
iramachio
イラマチオ
Irrumatio. Face-fucking.
shaburu
しゃぶる
To suck and lick.
- chin-shabu
  ちんシャブ
  Penis-sucking.
kuwaeru
咥える
To hold [something] into [one's] mouth.
- pan wo kuwaete hashiru
  パンを咥えて走る
  To run with a bread in mouth. (if you know what I'm talking about.)
gokkun
ごっくん
Gulp. (onomatopoeia.)
Semen-swallowing.
kun'niringusu クンニリングス
kun'ni クンニ
Cunnilingus.
- Not to be confused with:
- kan'ningu
  カンニング
  "Cunning." Cheating on an exam.
anaru-name
アナル舐め
"Anal-licking."
Rimming.

Manga: Inuyashiki いぬやしき (Chapter 18)

shabure......
しゃぶれ･･････
Suck [it].....
(meireikei 命令形 of shaburu.)

Millaarc ミラアルク doing a "chin peace" sign, ago piisu 顎ピース.

Character: Millaarc ミラアルク
Manga: Senki Zesshou Symphogear XV, 戦姫絶唱シンフォギアＸＶ (Episode 3)

Context: in some cultures, making a V sign on your mouth like you're spreading is associated with cunnilingus as it evokes such image.^(vice.com) In Japan, this is often not the case. A girl doing this is typically just striking a pose called an ago-piisu 顎ピース, "chin peace [sign]."^{(whynotjapan.com)}

姦

A lot of Japanese words for sex have kan 姦 in it, including some made-up slangs about some really weird stuff.

wakan
和姦
Consensual sex. "Harmonious sex."
goukan
強姦
Forced sex. "Strength sex." (strength = force.)
rinkan
輪姦
Gang rape. "Wheel sex." (in the sense of a group surrounding someone.)
soukan
相姦
Adultery.
kinshin soukan
近親相姦
Incest.
aokan
青姦
Sex outdoors. "Blue sex."
juukan
獣姦
Bestiality. "Beast sex."
shikan
屍姦
Necrophilia. "Corpse sex."
kikaikan
機械姦
"Machine sex."
shikan
視姦
Ogling. Leering. "Sight sex."
- Wait a minute, this isn't actually sex!

Some "grotesque," guro グロ, terms that include kan 姦. Obviously these only refer to illustrations, not real people:

noukan
脳姦
"Brain sex." Brain-fucking.
gankoukan
眼孔姦
"Eye socket sex."

レイプ

The word goukan 強姦 is the formal term for "rape" in Japanese. In the general sense, there's also:

okasu
犯す
To do something you shouldn't do.
To violate [a law]. To break [a rule].
To commit [a crime].
To rape [a person].
yaru
犯る
To do [a crime].
To rape [a person.
- A gikun for yaru やる.

And this word that won't air on TV:

reipu
レイプ
"Rape."

It has derived terms like:

reipu-me
レイプ目
"Rape eyes." (as in the victim's.)
- An anime term referring to the "vacant eyes" drawn in hopeless characters.
gyaku-reipu
逆レイプ
"Reverse rape."
- Same logic as nanpa ナンパ, "to hit on," in which you assume it's about men hitting on women, so gyaku-nanpa 逆ナンパ is "women hitting on men."
- Abbreviated gyaku-re 逆レ and gyaku-nan 逆ナン respectively.

Takakamo Shizuno 高鴨穏乃, example of vacant eyes.

Anime: Saki Achiga-hen 咲-Saki-阿知賀編 Episode of Side-A (Episode 3)

Context: "rape" eyes.

Manga: Detroit Metal City, デトロイト・メタル・シティ (Volume 2, Omake, DETROIT・MOE・CITY)

Context: death metal fans grotesquely misinterpret an innocent scene.
pyon
ピョン
*spring* (sound effect for jumping.)
oo'
おおっ
Ooh.
uo~~ chichioya wo yarou to shiteru~
うおーー父親を犯ろうとしてるー
Wow~~ [she] is trying to fuck [her] father!!
- ~shiyou to suru - "to try to" in the sense of "to seem to be going to."
gyaku reipu daa
逆レイプだぁ
[It] is reverse rape!

Also related:

chikan
痴漢
Molestation. (a crime.)
(male) molester.
chijo
痴女
(female) molester.

Counting

In Japanese, counting is done with rather specific, but also confusingly ambiguous counters. For reference, some words that can end up being taken suggestively:

The counter for penises, phallic objects, dildos, pencils, bananas, swords, strands of hair, arms, legs, and other long, cylindrical objects in general is hon 本.

ippon
一本
One [banana].
- banana ippon
  バナナ一本
  "One banana."
nihon
二本
Two [bananas].
sanbon
三本
Three [bananas].
nanbon
何本
An unknown number of [bananas].
How many [bananas]?

But wait, are testicles part of the penis? Because then it's not cylindrical anymore. Well, let's not think deeply about this.

The counter for vaginas, if we're talking about the vagina as an orifices, would be ana 穴, "hole." Just like the mouth, and the anus, are also holes, except when counting it's normally read with its on'yomi, ketsu 穴, because ichi, ni, san are on'yomi while hito-, futa-, mi- are kun'yomi.

ikketsu １穴
hito-ana 一穴
One hole.
niketsu ２欠
futa-ana ニ穴
Two holes.
sanketsu ３穴
mi-ana 三穴
Three holes.
nanketsu 何穴
nan'ana 何穴
An unknown number of holes.
How many holes?

The counter for ejaculations, shots, gun shots, hits, strikes, punches, etc. is hatsu 発.

ippatsu
一発
One shot. One hit.
nihatsu
二発
Two shots. Two hits.
sanpatsu
三発
Three shots. Three hits.
nanpatsu
何発
An unknown number of shots.
How many shots?

The counter for sexual intercourse, or the number of times anything at all is done repeatedly, is kai 回.

ikkai
一回
One time. Once.
nikai
二回
Two times. Twice.
sankai
三回
Three times. Thrice.
nankai
何回
An unknown number of times.
How many times?

The counter for sexual partners, or people in general, is -ri ～人 or nin 人.

hitori
一人
One person.
futari
二人
Two people.
san'nin
三人
Three people.
nan'nin
何人
An unknown number of people.
How many people.
- nan'nin mo
  何人も
  A lot of people. (more people than you can know.)
- If this is combined with yaru, you get this ambiguity:
- nan'nin mo yatta
  何人もやった
  [Someone] killed a lot of people.
  [Someone] fucked a lot of people.

In the case of orgies, there's a slang to count the number of people in it: pii ピー, which comes from the katakanization of the English letter P. This P can refer to the "page number" of a book, manga, etc. It can also refer to the number of "players" in a video-game. Which is where the orgy slang probably came from.

ichi-pii
１Ｐ
One player. One page.
ni-pii
２Ｐ
Two players. Page two.
- ni-pii karaa
  ２Ｐカラー
  Player 2 color. (a different color scheme for a same character, from fighting games where two players can select the same character. E.g. Bowsette blonde and red-hair versions.)
san-pii
３Ｐ
Three players. Page three.
An orgy of 3 people. Threesome.
yon-pii
４Ｐ
Four players. Page four.
An orgy of 4 people. Foursome.
go-pii
５Ｐ
Five players. Page five.
An orgy of 5 people. Fivesome.

Graffiti

Regarding body writing:

rakugaki
落書き
Scribble. Sketch.
karada ni rakugaki
体に落書き
Scribble on body. Body graffiti.

Since you could write virtually anything on someone's body, I'm not going to list everything you could write. Instead, let's focus on the noteworthy things:

Manga: Seitokai Yakuindomo 生徒会役員共 (Volume 1, Chapter 2, Page 16, 魅惑の入り口)

Context: the student council president comes up with an idea to make people want to put in suggestion into their suggestion box.
- meyasubako, by seitokai
  目安箱　ｂｙ生徒会
  Suggestion box, by student council.
omankomaaku おマンコマーク
mankomaaku マンコマーク
omekomaaku オメコマーク
The "pussy mark" refers to a symbol featuring two concentric circles ◎, one vertical line ｜, and ticks around the outer circle representing pubes.

Note to be confused with:

A "power plant mark," hatsudensho no maaku 発電所のマーク.