Thank you for the replies! And also for the links. I will check them out!
My husband is not a bad guy, and I hope I didn't make him sound as if he is. He didn't plan for this to happen, nor want it to happen, it just did. Sure, the timing sucks because this does all coincide with me being pregnant, but he has been supportive throughout and right by my side.
He never brought up his past poly relationship because, truthfully, it wasn't relevant at the time. It wasn't something he was hiding from me, it was just something that we never discussed. I do not have to have to know everything from his past, just as he does not know everything from my own. When he realized his feelings is when he told me about his past poly relationship, and his reasoning is to try to help me understand the workings of his mind and how it is possible to care deeply for more than one woman (or person) at a time.
He's trying to help me understand, deal with, and eventually accept what has happened. He is not trying to change my way of thinking or get me into a poly relationship. And to be honest, I don't know that M would be interested in a poly relationship, either.