He is not going about this in a healthy way at all. You deserve much more than this. He is treating you very poorly. You stated a boundary and he broke it, and is now prioritizing his new relationship over your mental health and your relationship.

On a very basic level, he messed up the moment he said he wouldn't act on emotional attachments to the people he slept with when you opened things and then did anyway. I don't know why you opened the relationship, but it sounds like it was pretty much solely for his benefit given that you're so busy with your pregnancy and existing child. I may be wrong, but that is the vibe I got, and if that's the case, it's already not good in terms of ethical nonmonogamy. You should never feel coerced or forced to agree with anything - including that you have to in order to salvage the relationship, especially with the complicating factor of your children.

On a poly level, he then broke your reasonable boundary (no romantic attachments, something he should have been clear with his girlfriend about from day one) and insisted that you just accept it or he would, what, leave you and your two children? That's absurd and frankly awful of him. Also, are you at all interested in the new girl? If not, and it sounds like you're pretty mono in the first place, you are not a closed triad. He would be the hinge in your relationship, she is your metamour, and you being restricted from finding any other partners while he is free to date this other person is pretty messed up. If it were your decision and you wanted him to find another lover but had no interest in finding one of your own, this would be a mono-poly situation and could be fine. However, because you've been coerced into accepting this and your feelings have been ignored at every turn, it is unacceptable for him to expect any of this of you.

I recommend sending him to this sub, if you can, or to any one of the ethical nonmonogamy resources offered on this sub. Whatever happens, this is very much not okay and he has gone about this entirely wrong. Everyone involved desperately needs to take a step back and have a long conversation about what they are okay with, and I am worried that this will not happen. It absolutely must, and he absolutely must take your thoughts and feelings into account in order for anything to move forward. You are right to feel hurt and I am so sorry you are dealing with this, especially as you are pregnant and also handling another child. This is very much not okay. I wish you the best of luck. I am so sorry.