Getting cheated on should only piss you off if it kept you from dating someone else and wasted your time. If you thought she was the one you were gonna marry then its disappointing but whatever. Move on. Every relationship I was ever in I basically ended up looking forward to the day we broke up, except of course until I met my wife. If I found out a girl was cheating or said she wanted to have a break and see other people etc I kinda got excited. Like ok this is over? Cool see ya. I get not everybody thinks that way but dating to me honestly was about having fun, maybe making a few long term friends, and mostly just not wasting time being young and not getting out etc.

So yeah, fuck it. She wants to cheat on boyfriends, who cares. They're boyfriends. It's not the end of the world. Eventually she might meet a guy she sees herself staying with long term and she won't want to cheat. That's how it works with these kinds of women. They're like monkeys. They need a new branch lined up before they leap. If you find yourself in a relationship with someone like this and realize it, dont get hurt. Just accept the reality of it and move on. Its all meaningless bullshit until we die anyways.