yaranaika? やらないか？

On the internet, yaranaika is a meme based on a certain gay manga related to faces like this:

Character: Jashin-chan 邪神ちゃん
Anime: Jashin-chan Dropkick, 邪神ちゃんドロップキック (Episode 4)

• Meaning

Meaning

In Japanese, yaranaika? やらないか？ means "won't [you] do [it]?" in this case in the sense of "won't [you] have sex with [me]?"

It's the verb yaru やる, "to do," plus the negative -nai ～ない suffix, plus the doubtful ka か particle.

This phrase, and the yaranaika face, comes from the bara バラ manga Kuso Miso Technique くそみそテクニック.

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]?