Confused with what my ex wants

I broke up with my ex last night because I found out that he has been talking to this other woman (who used to be my friend) who have previously 'ruined' our relationship. I'm monogamous, but my ex has always wanted a multiple partners for himself and that he secretly has always wanted for me and this other woman to be sister wives. I explained to him that I only want two of us only, he agreed to it and told me that he would cut this woman off in regards to our relationship.

I found out that he met her secretly behind my back, he lied about not wanting to be with her anymore. He admitted that he likes her because she understands his vision and they share the same interest (video games) that he doesn't share with me. I couldn't take the lie, he didn't stick to promise. He told me that he has warned me previously that he wants to be able to be with other women, and that I ignored this 'flag' so that what he was doing behind my back is justified. I think he was right and wrong at the same time. I should've ended the relationship right there and then.

So I told him why don't he make it official with that other woman? He told me that she doesn't like him that way, and that she likes me more than she likes him. Apparently this other woman has been asking a lot about me to him, making sure that he's treating me good and that he makes me happy.

And I told him if you really want multiple partners, why don't you seek out for people who are poly, who are open about it. He told me that's not what he wants and I don't understand his vision. I also asked him if that's the case, we can just stay together, I will let him have multiple partners as long as he let me have the same thing. He told me to fuck off.

I am so confused.