It's hard to describe this one, so it may come out a bit vague.
Boy and girl each have significant others. They are *possibly* at a party and then leave when their boyfriend/girlfriend don't show up.
They end up hooking up. I think it turns out that the girlfriend broke up with the boy and he had always found the main girl more attractive anyway.
Yeah. Vague. Sorry.
Any help is appreciated. I read it on Fakku many years ago. Before pay to play.