Men have affairs with married women, but not with anything close to the frequence and naivete of women having affairs with married men...
So remind yourself of this: "nice and wonderful" men don't cheat on their wives. He may be telling you the truth, but he's lying and sneaking around at home.
Married men can be so charming; they leave all their nastiness at home with their wives and you only see the good side. I've had a couple of those kinds of relationships myself. Both times, I wound up broken hearted and the man stayed married.
If you're smart, you'll learn from my experience, and get out now. If you're the kind of person who only learns from her own experience, you'll probably hang around until the bitter end -- until his wife finds out, or until he finds another playmate. Meanwhile, you're facing years of being alone on holidays and handling crises without help.
Of course, you're probably hoping he's going to divorce his
wife and marry you. But is that really what you want? To switch roles and
become the one he's lying to while he's having honest pillow talk with
someone else? (Believe me, men rarely change their patterns.) Even if he
suddenly becomes true-blue, you'll never have him all to yourself; he'll
have lifelong obligations to his former wife and family.