Knowingly sleeping with a person in any kind of monogamous relationship is fucked up. I had a friend who messed around with a dude who was unsatisfied sexually in his relationship. So instead of talking with his girlfriend, or even breaking up, he sticks his dick in the first girl he finds. I was disgusted at him, but even more so at my friend, who enabled this shitty behaviour. She's also the kind of person who is suuuuper insecure about guys. She had a crush on that guy for a while, and kept badmouthing his poor girlfriend, parroting whatever he told her. Almost ended our friendship.
So uh, kind of parroting what cara said, but if you knowingly stick your genitals in or on someone else's genitals, fuck you. If you're unhappy in your relationship and don't have the guts to talk or end it, fuck you too.