Sex also plays a key role. If he is not getting any action from his habitual lady, he will look elsewhere. But, just because he says he isn't sleeping with his wife, doesn't mean that's not true. You can't trust anything a cheater says because his actions are already speaking louder than his words.
The characteristics of the mistress are usually the same. The mistress is the pretty, fun, and successful girl. She is a joy to be around, while he has a martini drinking, stress inducing, non-sexual woman back home. When he's with you his problems back at home fade away.
As soon as you find out there is someone else in his life move on. What if he was your boyfriend and doing this to you? If he cheats on his girlfriend, more than likely it's not going to be hard for him to cheat again. And even if he does leave his partner to be with you, somewhere in your mind you must always wonder if he will do it to you, too. And if so, this time you will be the one hurting. The mistress rarely wins.