I don't normally reply to many posts - mostly lurk - but I really felt I wanted to here.
OP I don't think YABU really, but I do think you may have to be sensitive towards the new GF's feelings a bit on this one.
Isn't what the OP and her ex have actually the model of 'co-parenting' - the holy grail of parenting after a split??? The fact that they are friends, can get along, can both jointly be there to support their DD and spend time with her to make her feel secure and cared for should be highly praise.
My DP had cause to go to a court mandated parenting class with his Ex a short time ago. Long story and I won't bore you with it here, but they bought back some information. The relationship the OP describes was the exact thing that was being encouraged here - the thing that they were told was in the best interests of their child.
So no, I don't think OP and her partner are being unreasonable.
It also does sound like OP has made efforts to include new GF in this, and that so far she has declined. We only have OPs word on this, but it seems very reasonable to me.
However, OP, a new baby does bring change for everyone involved. And because of your close links with your Ex that includes you too. He will need ot be there for his partner and their new baby and that may mean a reduction in the time that he has to spend around your house in the evenings etc.
Changes will have to be made, and I'm afraid you may need to accept that this means it somewhat curtails his time spent around you. Your Ex's priority at the moment is his DD. In a few months time it will be his DD AND his DS, and he will have to manage that.
Also, she is pregnant. Hormones made me ridiculously sensitive and insecure. Please give her a little bit of leeway.
As for what you can do about the situation, well it's kind of your Exs decision to make really, so out of your hands. It is for him to learn/ decide how to manage his competing priorities.
The only thing I can suggest you do is have a chat to him and let him know that you are proud of the co-parenting relationship that you have developed, that you think is i very good for DD, and that you would like it continue and would very much like to include his other family too. Also raise how you both might manage any changes post birth with DD and how you will broach it with her. Then really it is up to him to make his decision as best he can.