I feel like all of these people, who all talk so wonderfully about The Ethical Slut, need to sit down and copy it out on a chalkboard 100 times. No matter what pregnancy hormones may be, you cannot reasonably tell me that what is going on is ethical or right.
Well, I'm a lot newer to poly than you are, so I hesitate to give advice. But here goes, anyway, for what it's worth.
I do know something about ethics, but I don't even need to draw on that to figure out your boyfriend is being a jerk. His conduct is irresponsible, even reprehensible, and there's no reason you should have to put up with it.
If I read this right, he gets to do whatever he wants, and he resents any effort on your part to establish boundaries; but then, he gets resentful and jealous if you try to establish the kind of close relationships with other people that might offer you support and comfort through your pregnancy. So, you are stuck at home, without other support, while he goes sneaking around at will. He wins, you lose.
Am I reading this right?
I don't know exactly what to advise - others on this forum would probably do better - but it seems your boyfriend needs a good talkin' to, some hard and direct conversations about the nature of your relationship and the commitments it entails . . . especially since you're pregnant with (I assume) his child.
Starting those conversations will be a fearful thing, but at least it's a fear you'll see the other side of. It would be better than the constant agony of anxiety you're feeling now.
The bottom line, the ace up your sleeve, is the possibility that you might actually be better off without him. True, you would risk being alone for the rest of your pregnancy, but you're alone at least half the time, now, and, when you're not alone, you're dealing with his deception and his petty self-absorption, which is a source of stress you really don't need.
Without him, you might be more free to develop a support network for yourself, at least a circle of friends who can keep you company through the coming months, or maybe even an intentional community you can join.
I don't know if that's feasible for you, financially, but it wouldn't hurt to expand your options.
Anyway, with that up your sleeve, you can be more direct and forceful in your conversations with your boyfriend. It takes away some of the power he has over you.