I"m not sure I've ever understood why a woman wants a man who cheats on his wife. Surely you understand that if he will cheat on her, he will also cheat on you. He will never be yours for real. You know what kind of man he is; He isn't a man you can honor and trust.
If he's incapable of being honorable, consider being so yourself. Get out of this relationship and look for someone who is single.
