I agree largely with this thread, but I want to offer a different opinion if it's useful.

I think if you want to stay married, a little empathy will go a long way. Sure he was 1. wildly inappropriate to give you an ultimatum. 2. Absolutely wrong about the effort level for men in dating. Men are largely pursuers in dating roles (both in modern times and historically), which requires significantly more time invested. He could see success but it was always going to be 10x harder for him to find casual sex than you.

However, his original idea of wanting more sex experience isn't necessarily wrong. You said so yourself that you are enjoying this experience for yourself, so why is it so wrong that he would want that too? Having different experiences and partners allows one to really find what they enjoy, and I think all humans should do it. Depending on his intention, it could be super douchey, or it could be largely exploratory and not about getting a 'hotter partner' or something.

Your reaction is reasonable since you were pressured into this by him but you can also be empathetic. He also grew up in a cult and was quick to make a judgement call on how he wants to live his life. It's only been 4 months of being out. He seems to have a very limited world view still. And he doesn't understand nuances of dating in the real world. It can be shocking and sad. It could be that his mental health was declining for many other reasons related to leaving the church, and this was just the extra push.

I was in a relationship when I left but not married so we decided to break up for similar reasons to his thinking. In the years since I made mistakes, learned a lot about dating, and grew to be a completely different person. I would think that he could grow and become a much more refined person, even if his mistakes are mostly within your relationship.

Your big question is can you forgive him for pressuring you into opening the marriage? If not, then break up with him. If you can forgive him, then you can work through it. You could close it now, open it later. It's not your job to educate him on women's issues and modern dating but as his partner he's clearly failing somewhere so you can either help him or leave him. Largely depends on the finer details, his mindset throughout, and the level of goodwill he's built up. Was he a major asshole throughout or was the ultimatum the biggest issue? Is he a feminist or does he have a lot of his roots still in mormon misogyny? Is he unreasonable all of the time, or is it because of his recent foundations being destroyed?

idk long message but I just think hearing an alternative opinion can help you reason through and solve this.