I'm sure you hoped that your husband would take the impetus to change from your ultimatum (please correct me if I misunderstand that's what it was you gave him), rather than choose to leave.
I didn't give him an ultimatum per se, but I sure didn't volunteer to break it off with my girl either. He kept telling me he'd "find a way to be ok with it" which so far, meant a strict DADT policy where I was not even allowed to speak her name, then him freaking out hardcore everytime he realized I'd hung out with her.
Though I've always openly identified as bisexual, I don't necessarily consider myself poly though I was in a several open relationships before my marriage, and was the "unicorn" in a couple of triads. Those were pretty much free-for-alls which left me feeling used and empty. I wasn't out to open my marriage, but I did go out and flirt a lot, and I just couldn't deny my body-mind connection to this particular woman when she started flirting back. She identifies as poly, which I totally get, but I'm reluctant to label myself anything right now while I'm pretty much in a free-fall.