Dude you had to see that coming. It was a MAJOR red flag for her to even confess that she was interested in you while being in a relationship along with other things. Morally you getting with your friends’ girl is seen as wrong, but you are right. You are an adult and what you do with your relationships are nobody business. Next time, make sure to be clear with your partner what you’re looking for even if it’s long or short-term. It’s not guaranteed they won’t cheat, but at least you were clear about things. I don’t think you were terribly wrong, so don’t overthink too much about it. Stuff happens in life, and you can’t let it weigh you down.