She's not giving us that option right now.
He's not giving you that option. He's letting her decide how it's going to be. It's not her call unless he lets it be her call. He is letting her decide and letting you deal with it instead of owning his own part in it and making a decision like an adult.
If HE can't convince her to "give poly a chance", and he's the one she is in a relationship with, what makes you think YOU, "the other woman", can convince her? She sounds like a classic "cowgirl".
You are still young. Your life is not over if you break up with him. At least you don't have kids, or you'd be all in a wad about that too.