Ugh, he's acting extremely selfishly. I'm with RedPepper, this is the time for bonding with the new baby. If contraception failed (or you both made the choice not to use any), this child is just as much his responsibility as it is yours.
He just wants to go out and shag whoever whenever. You'd planned a nice evening at home, and instead he made a date and went out? Unacceptable, and not good poly practice.
You've been through a divorce and pregnancy and birth and now you have a newborn!
I just want to say, dump him. He is not acting in a loving respectful manner at all. Go home to your parents, take care of your daughter, and make sure he's giving you child support. You were both swept up in NRE (new relationship energy), moved in together way too soon, now his lust for you has lessened and he just wants to cat around. You and your (plural) child are cramping his style!
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 3 years