When it happened: Season 4, Episode 1, "Iowa"

What made it hot: OK, to be honest, most of this was not hot, as Jezebel so deftly pointed out. Desi really goes to town on Marnie's ass in a way that makes you borderline nervous for her. It's the fast, uncontrolled motions of a first-timer who's just excited to be invited to this thing. But it did put ass-eating on the plate for straight people, so to speak, and therefore it deserves a mention.