It’s an OLD inside joke from 4Chan, if not earlier. Somebody comments something about Vanilla Sex (lit. “heterosexual sex in the missionary position for the sole purpose of reproduction”), and everybody in the know replies with some kind of disgust or derision. Many lols from confused newbies that aren’t in on the joke. It’s along the same line as those “I bet you shower naked” replies on thirst pics on tumblr/facebook/whatever.