The whole taking care of THREE children bugs me. What exactly does your husband do to help you out? If he wants to have another girlfriend, he needs to first be sure he is taking care of the family he already has.
My husband is like a child sometimes too. Often, actually. Since I stopped working I've been doing everything in the household, which I truly don't mind. But it's when I give him that one little thing I need done and three days later it's still sitting there and then I do it finally and he goes "I was just gonna". Ugh. Today I actually finally let him have it and am boycotting doing anything else I have asked him to do because I don't ask for alot. (Damn YGirl, it must be my darned bipolar acting up again, lol) I'm hoping that it makes him realize he has some responsibilities around here too and I cannot do everything, especially with another child on the way. We'll see.
A few people are talking about trust issues in regards to his feelings for his ex being the apparent catalyst for the poly discussion. I feel a little differently on that. He didn't physically act on his feelings for her behind your back. And, from what you say, she herself wasn't aware he had those feelings. The way I see it, he didn't break your trust because he didn't fall for her intentionally nor act on these emotions without first talking to you when he realized he did have the emotions. Have you never had a friend you suddenly realized you felt more for without the intent to ever do so? It may be that these feelings and the confusion they caused are what led him to realizing he was poly in the first place though he was never looking to be. I can't fault him for that as I think many of us discover our poly nature by actually loving more than one person simultaneously without the intention of doing so. I have to commend him for being honest with you about it, even if he was a little late in telling you the why of it.
What I do not like is his yes/no line of questioning. Until everything else clears and you feel secure enough to give a definitive answer, I think you need to let him know he'll have to settle for a maybe right now. It sounds like you two have enough to handle on your own before adding another person and all her issues/compromises/needs to the plate. Not to mention all the compromises their new relationship will have, including time constraints and effective communication skills. He should not be putting all this pressure on you, but should instead work on more effectively handling his role as husband and father in order to build your trust and faith in him being able to handle more in his life.
You ARE amazingly open minded. I wish you all the best.
I'd rather be hated for who I am than loved for who I am not.