Before I write, can we get a mono's perspective here? I feel way to attached to your situation as a poly bi male who's been in your situation to a lesser extent to react to your predicament with objectivity, and clarity.
As i've said, I have been in your situation before to a lesser extent. At the point where your partner brings you an ultimatum, thing have gotten to the tipping point. Since he was the one who presented it, what's going through his head is most likely something more extreme than any of us can imagine. To to be married for almost 3 years and say something like,
"get on board to getting back where we were or move out and get an apartment and be fine"
He's got to be at an extreme emotional place right now. When an ex gave my current g/f an ultimatum like that, he was literally having a psychotic breakdown! The world he lived in was crumbling because a polyamorous relationship was too far beyond his boundaries to consider.
I know that some people mentioned above, he seems to be throwing a fit because his other relationships failed; but i'm not convinced that's the case given your current information. What are the details of his relationships? It seems logical to me that he was only dating other people to adapt and get over what you were doing. However he never thought of it the same as you...ever, hence his "thoughts" on the girls he was dating and maybe how he never seemed serious about them. I would agree that this is his own fault, and most likely like a lot of men, he has some deep seated objectification of women issues. His logic in dating other people was seemingly far different than the reasons you are with cue.
I wish I could give some definitive help but to be honest, but I just can't. IMHO, I think you're at a different "level" in your state of being, than he is. I don't want to say "higher" but...yeah. It seems all you are doing is trying to compromise. However this is just my opinion, and we obviously don't have his side of the story. Maybe he could write on here and get others' opinions?
god, my heart goes out to you. Keep us updated!
peace & love