I knowingly got involved with a married guy when I was 18. He was a complete and utter slut, one of those men who just fancy everyone, all the time, and aren't even vaguely subtle about it. I did feel badly about his wife (who I never met), but at I was 18 and bonkers.
His wife had caught him cheating before, a few times, and stayed with him. I don't think she ever found out about us.