It wouldn't be a good idea for the baby, if no one else in that kind of situation. A child should be raised in a positive and supportive environment. Why would someone want to bring a child into a situation where the adults were unhappy and confused?

Everyone needs to be honest with themselves and each other, and move forward. They need to be strong and make decisions and settle it before the baby gets in the middle of everything.

As far as the adults are concerned:

Why would a man want his wife to stay in a relationship with him, that is supposed to be built on love and trust but is now for the sake of a future child, when he deserves to be with someone who is with him because she wants to be?

What does the mother gain by staying with the father that would be better than being stable on her own and letting this man move on, and have a relationship with his child that doesn't have to be under so much stress?

And if the woman wants to be with this other man, I'm assuming he wants to be with her as well, and why would they want to be separated and keep everyone in a situation where no one is happy?

It sounds like it would be a good time to involve a family counselor and have everyone state their intentions in an adult manner and make a plan.

I hope that all of these theoretical people will find peace.