Dude idk why so many ppl are getting butt hurt from your post, I completely agree with you here... In my mind if a chick is in a relationship it is entirely up to her she if she chooses to cheat or not, and because of that you're not at fault. Clearly their relationship isn't even that healthy to begin with if it comes to that anyhow. As long as you have good intentions or make your intentions clear to the chick, then the balls in her court and she is free to do whatever she wants. Like it pisses me off when guys who get cheated on gets pissed and instead of taking their anger out on their girl who did it, they go after the guy as if he FORCED her to do something she wanted to do. I mean that being said you ever right to be pissed, just at your gf who betrayed you. The guy who she cheated on you with owes you nothing whatsoever.
Only times when I think this is huge dick move is if you do it to a girl who is dating one of your close friends, then that is just a betrayal right there to your friend. Or if the girl who cheats with you thinks you have different intentions than you actually do. Otherwise everything is fair game, she isn't married yet.