My last girlfriend fooled around with a mutual friend just before summer break last year. It wasn't too serious a relationship, so it didn't bother me personally. And actually, i didn't necessarily break up with her because of that itself, though it is what spurred our breakup. Really, to see her beat herself up so much and want so strongly for my forgiveness - and thus continue the relationship - i realized that she was looking for something much more serious than i really wanted.

So that, paired with cheating being a bad move on principle, brought me to end things with her. The cheating in itself, though, never did get to me. Which isn't to say that it shouldn't, because cheating is never cool. i guess my experience just happened to be an odd one.