In pretty much any /r/AskReddit thread about shitty relationships, there's always comments like "She cheated on me with my best friend", or "After we broke up she slept with my best friend the next week". Recently, it happened to one of my friends and so I started wondering, why does it happen so frequently?

Does the girl have the goal to hurt their ex as much as possible? Or is it just the sexual tension that built up because sleeping with him is "forbidden"? The reason can't be that she just wants to have sex after the break up, as she could choose countless other guys, and she'd be seen as less of a slut by the community if she fucked some other dude.

What's your opinion on this?