It sounds to me you know already that you strongly prefer monogamous relationships. It's just this one guy who is so awesome makes you wonder a bit. Read what he gave you and think about it but it seems from your post that you know what you truly need and want.
He's married and poly. Neither of those are likely to change. You need to have a connection with one person who also connects on that level only with you. He can't give you that ever. You also can't be what he needs, a poly person who is fine with being a secondary relationship. It's time to walk away from each other while it is possible to do so lovingly.
You have reached this conclusion on your own. You may well need a break from being around each other as you adjust to being friends. I am restricting time with a former lover as I adjust to being friends only. I also meet very few people that I can fall in love with. Maybe 4 or 5 so far (I'm 40).
Also if he is the man you believe him to be, he will understand. He will be sad and unhappy, no doubt, but I suspect he knows already you and he are not compatible for the long term.
Think perhaps about what your relationship with him brought, what you learned about yourself. Maybe you learned that you can survive loving and losing that love.