Welcome MRC from a fellow New Yorker.
Sounds like you two have a bit of a chore ahead sorting things out. I don't hear any mention of a 3rd yet. Is he anticipating desiring someone and just being proactive by declaring his poly-ness? Or is there a 3rd you haven't yet spoken of yet. Regardless...you will get some very good advice and feedback on these forums.
My wife is the poly one of our relationship and I can relate to some of the issues you have stated. Making (and following) rules is important for BOTH of you. This is a two-way street, and he needs to listen to your needs as much as you need to listen to him.
I see this as a big red flag. Bringing anyone into the world is a huge deal. You two really need to be on-board any decisions in this matter. He needs to take your concerns much more seriously than he seems to. Especially since a child would bind whomever you meet permanently to your lives, and so early in your poly-adventure...seems dangerous to me.
- Your desire/needs for companionship
As the "mono" one in my relationship (my wife has a BF), I also find there to be a desire for someone else in my life. Does this make me poly? I don't think so, but I don't know. We've even discussed this. I feel that I wouldn't see another person in my life even remotely the same as I see my wife. But at this time she would have an issue with that. Regardless of my own personal dilemma, I wanted you to know that from my perspective (which I see similar to yours) I know where you are coming from.
Bottom line (and you will hear this often), keep communicating. Don't let a few failed discussions get you down. To make any "normal" relationship work takes alot of effort, yet to bring the poly experience into a relationship requires a strong base and much communication.
I wish you well.