A woman who has a sexual relationship with a married man is called a mistress, right? But what do you call a man who has a sexual relationship with a married woman? I would like to know formal and informal ways to call such men in that regard.
A mistress tends to be a woman that is "kept" by a man and implies a degree of inequality - ie the man has a double-life and the mistress is kept waiting. He may even be financially supporting her.
There isn't really an exclusively-male equivalent. "Lover" can apply to either sex in an affair, but equally two people in an honest relationship can also be called "lovers" without any negative connotation. "Paramour" can be used for either sex and is more exclusively used for illicit relationships, but is not widely used.