I'm sorry you struggle.
I have had zero interest in sex and that's a few years now. My husband is very sexually driven so it's been an issue with us for some time. He seems to think bringing in another person will fix me
Is he expecting that you having a female lover will make it so you have interest in sex with him again?
Is he expecting that you will bring home a female lover for BOTH of you to share group sex?
What happens if you Open so you can explore your bi side and you still aren't interested in sex with hubby? What then?
What would you do in my situation?
You don't seem to want to Open while pregnant. So don't. Instead, spend some more time thinking things out.
Poly isn't the "bandaid" to fix what is broken in a marriage. It's just going to magnify all the cracks that are already there.
These are his children too... is he helping you enough? Are you burning out on pregnancy/early childhood care so there's no energy left at the end of the day for couple time or sex? If so... can you get house help? Can he pitch in more?
I feel bad for depriving him of intimacy which is a huge thing for him
It is your body. You determine when/with who you share sex with your body. Partnered sex share isn't a "right" you are taking away from him. It is a privilege.
It isn't like you are a toaster that dispenses toast whenever he wants some.
so introducing another partner would be nice for him and may end up easing some of our current issues but not sure.
If the "goal" of poly is him to have sex with someone else so he stops pressuring you for sex?
- You could ask him to stop pressuring you for sex. That's the most direct route.
- You could think about him dating someone else on his own. That might solve it indirectly... or he has a GF and he still pressures you.
it would kill me watching them connect when I really don't feel like I can
Him dating someone else doesn't have to mean group sex. You also don't have to be watching him and his future GF having sex. Is that what you meant?

If so, you can say NO to that. Your consent to participate in something or not belongs to you.
Did you mean you are worried about him dating someone else in general? You are ok dating others for you, but feel weird about him dating others? Could work to overcome that so each of you can date separately when you do Open.
You don't seem to want to Open while pregnant, so wait. And in the meanwhile, educate yourselves and get to the root of what you think poly would "solve" for you guys. Then solve it before you actually poly.
It's not your future dating partner(s) job to be your "marriage bandaid."
You might also consider if this goes beyond pregnancy hormones.
We crushed on each other through high school and got together during my last year. He's been with other people before me, I've only had superficial relationships and no other sexual partners. We've always been mono, but I've known for a long time that I'm bisexual.
Is it that you guys got together and married too young? And now you've outgrown him? But aren't ready to think about divorce?
I'm sorry you are struggling. But I think rather than rushing to Open so your husband can have sex more often... you could spend some time thinking about what it is your really want from your life and prepare for the upcoming birth first. Take it one thing at a time.
Maybe these help you in your research:
http://openingup.net/resources/free-downloads-from-opening-up/
http://practicalpolyamory.com/downloadabledocuments.html
http://www.kathylabriola.com/articles
https://www.morethantwo.com
Galagirl