My girlfriend cheated on me, and she greatly regrets it now. She was feeling unwanted by me, somewhat understandably so.
However now, I find I get turned on by the idea, but I also feel really bad about it later on.
She doesn't like that I think about it as a turn on either. I think part of the reason why I found it a turn on is that I always wanted her to act more "sl-tty" in bed, and to me, what she did is sort of like a fantasy, except the fantasy happened to someone else, which makes me feel terrible. I never had fantasies like this before - certainly of "* * * * ty" girls, but not of my girlfriend with someone else.
I broke up with her, because I didn't want to feel so bad anymore, but she wants me back.