I'll confess I'm really confused by these polyamorous situations.
I think it's generally the position of most here on SI that there's no statute of limitations on a betrayed spouse's grief and trauma, and that it can come roaring back years or even decades later. It's also true that anyone can divorce for any reason, although some reasons are objectively better reasons than others -- such as betrayal via adultery.
If I understand the situation, you blameshifted onto him after D-Day (and maybe DARVO'd him too, if your hint about "emotional outbursts" is perhaps a clue?) and then you both rugswept the affair. Maybe trickle truth, too? You don't say, only that you didn't handle D-Day well or the subsequent months. I can only say in my own circumstance, my wife also didn't handle the "subsequent month well" and it did incalculable damage.
I myself am nearly four years out and participated in rugsweeping my WW's affair. She did her fair share of blameshifting, rationalizations, trickle truth and lack of transparency I won't dive into here. But it led me to the place four years later where I've told her I definitively want a divorce and I'm working actively to bring that about now.
I don't think anyone would say that I'm somehow in the wrong for now moving forward with divorce, or that I misled her. Indeed the vast majority of folks on this website seem to think I should have done so earlier, and that I've given this attempted reconciliation every effort.
I'm not at 10 years out, but I can see how I would continue to be triggered and traumatized by the situation that far out, if somewhat less than now. In fact, I'm sure I will be, guaranteed, even after divorce. if I had continued to swallow my pain, I could definitely see a scenario where in four more years or five more years from now, I would be a shell and just as completely done as I am now if not more so.
If I also understand your situation, you broached the idea of an open marriage/free pass but he initially shut that down. Then when an opportunity presented itself years later, you actively encouraged it and then participated in it joyfully.
So then "weirdly, somehow, it turns into a threesome situation. I cannot even explain how." Ok, but I think with a little more self examination, you could probably dig down and explain to yourself and perhaps to us something a little more detailed than "weirdly, somehow" -- frankly, that sounds a lot like the typical elision we read and hear from WS's all the time.
So if I'm keeping up with the situation here, you betrayed him, he was traumatized by the initial betrayal and your subsequent actions and words, you both rugswept, you offered a free sexual pass, then 8 years down the road he takes you up on the offer after additional cajoling on your part. Then he sees you not only encouraging but also actively and willingly and jubilantly participating in it.
Am I right so far? And that then developed into a polyamorous relationship in which you both took part. Then, predictably, as they seem to in these situations dangerously toying with the human heart, things turned dark.
He started developed independent feelings for her. You felt threatened. You also had feelings for her.
It ended, sort of messily and abruptly.
Then it started up again, and you said you didn't want it to be as "intense" as the first time, but that seems a pretty vague word, doesn't it?
What does "intense" mean specifically?
Regardless, things were "intense" the second time around --and somewhere along the line he then he invoked an in-home separation.
And now he wants a divorce.
I understand the "on this hand, on the other hand" viewpoint of this -- but what I don't understand from some of commenters here is how the onus for this is on the spouse who was originally betrayed. If I'm doing a thought experiment and reversing the genders entirely throughout, I can't see how I'd blame the original BW in this situation either. In fact, I wouldn't.
You betrayed him, handled the fallout terribly, may have even tried to DARVO or gaslight him, rugswept, and then indicated by the offer of a free pass that you saw sex as sort of a transactional act.
That damaged him beyond the original betrayal, in my view. Then he viewed the limping wounded marriage as a kind of half baked commitment with a lot of rugsweeping, and finally took you up on the offer of sex with another woman after you re-introduced and encouraged the idea.
He's not wrong, as you say, about your outbursts or about your lack of care for his feelings.
And now he's done.
Anyway, did I miss anything?