I'm so sorry this happened to you! You were just starting out with this fellow and you hadn't tried poly yet when you got pregnant. I'm pregnant now too; I know what it's like. It's so tiring, and you were going through a divorce, and you didn't feel totally supported by your partner.
So first, big hugs. Congratulations on the birth of your daughter.
Your bf is pushing you into dating other people because he wants to be able to date other people without feeling so guilty about it. But he's missed that the important thing isn't that you have another guy as a distraction. The important thing is that you feel secure and loved. A new boyfriend for you doesn't "balance out" a new girlfriend for him. A new girlfriend for him can only be balanced out by his actions towards you: being considerate, being open and honest, spending romantic time with you, spending time being a dad to give you a break, etc.
He's not doing that, is he? He's just adding extra stress onto a new mom's life and expecting you to deal. Tell him to come read our replies and shape up. Poly isn't impossible for you two, but he's going to have to be a little gentler.