Welcome to our forum.
I wonder if you and your husband don't have different wants and visions for how this polyamorous life is going to play out. It kind of seems like he sees the two you dating different people separately, whereas you rather hoped and expected that any new people you dated, you would date together as a couple.
Does your husband even see the arrangement as a DADT situation? I'm wondering why he didn't tell you about it when he first started sleeping with this coworker of his. Did he know you wanted "couple dating" and he didn't want to admit that he wanted something else?
Honestly, couple dating is usually hard to do. What are the odds of a person falling in love with one other person, and then multiply that by the odds that a person will fall in love with two specific people at the same time (and at the same level?).
Regardless, I think you and your husband will need to sit down and figure out if your respective polyamorous preferences can be compatible with each other. Can he agree to couple dating? Can you agree to individual dating? What compromises are there, and will they work?
And then, what do you do if you find out your poly paths lead in two very divergent directions? What if there is no room for compromise? I don't like breaking up and I hope it doesn't come to that, but the big question is, Are the two of you going to be happy together, with whatever polyamorous conditions you decide to implement?
It doesn't sound like going back to monogamy would work for either of you.
My next question concerns the kids; you are caring for three and a fourth is on the way. Could be difficult, going any further than that and trying to manage the complexities of polyamory as well. Kids take up lots of time, and well, polyamory tends to take up a lot of time as well. So your time will really be at a premium. You'll need a calendar.
Okay so given all this and I know it's personal of me to go here and likely none of my business, but can I suggest that four kids will be enough for you guys's situation? Might be time for hubby to get himself fixed, you know what I mean? that or at least birth control until you have a much clearer picture of what your poly future's going to look like. Maybe you'll have enough extra poly partners to help with the extra parenting? Then of course you also have to consider that one or more of those poly partners might want kids of their own. Sometimes you just have to be practical about how much of a load the family can handle.
I assume being far-along pregnant makes it tough to pursue any dates of your own. Which probably adds to your sense of unfairness that your husband is so free to pursue his other gal.
When this next child is born, things won't get much easier. I know you know all too well how much work it is to take care of a newborn. I expect your husband to step up to the plate and help out with that job, it's pretty much a necessity.
I think it must be hard to get a babysitter for a newborn, so I feel like you're going to have to put some of your poly plans on hold for a few years. Will you be able to tolerate your husband continuing his new relationship during that time? I can't imagine that being an easy decision to make.
The best you can do on our site is engage in a lot of reading, and post your thoughts, questions, and concerns as they arise. I think the Life stories and blogs board
might be especially helpful to you: real stories of real people figuring out how to make poly work. I'm certain lots of those stories will feature married couples with young children.
I sincerely send you my best wishes, and hope the bumps in the road get smoother.