Take him out of the equation for a moment.
What do YOU want? To marry? If so? Do you want a husband that is dating other people? Or a husband that is exclusive to you?
If exclusive to you -- best to end it here with him and be friends.
You have different beliefs. Neither is better than the other -- just different.
You have the right to seek what YOU want in Life. Do not compromise yourself.
If you are willing to let go of "exclusive" and come to terms with his poly side and be a monoamorous person in love with a polyamorous person?
Well... What's he mean you are the "one" -- the legal wife? The primary? What kind of open relationship model
does he subcribe to? How well do you deal with jealousy?
There's stuff to consider. Because maybe you are a monoamorous person who could maybe deal with a "V" with just one other person but NOT a large polytangle or a casul sex swinging scene where there's lots more numbers than just one other person besides you. Then there's the "being in right relationship" -- are you exclusive NOW? As BF and GF? With the LDR thing? If that is the expectation what's he doing with other people? Were promises broken or not?
You are in a time of discernment for yourself. But again -- there is NOTHING wrong with wanting monoamory in a monogamous relationship! You have the right to live your life as you most want it to be.