I had a girlfriend when I was in my 20's that would go out once or twice during the month and cheat on me. She was always very horny (and wet) when she'd get home, and she always demanded that I eat her out. I couldn't figure out why it turned me on so much, but it did.
My girlfriend would purposely go out with her friends to a black nightclub and each one of them had side black guys they would hook up with. I let her do her thing knowing he was sneaking by after I left on Sunday nights