I'm not feeling you on this one. It's not your place to play relationship sheriff. Who decided the information was "need to know"? If he has a baby girl on the way, what is more important to that baby girl's upbringing? her family staying together, or what her dad does with his penis? It's best for you to just mind your business with things like this.

I myself would be pretty pissed off about someone meddling in my business, whether i was in the wrong or not.