This is sexual harassment plain and simple. A man continually pressuring a woman for sex after she's said no is sexual harassment whether he's her partner or not, just as if, god forbid, he ever flat out ignores your no it would be rape, whether he's your partner or not.
Your body is YOURS, not his, and if he doesn't get that then it's perfectly acceptable to get "mean" about it.
If a partner were treating me like I was a sextoy without a right to my own feelings and person, I would end that relationship. At the very least, if I were you, I would tell him that this issue might well end your relationship (unless you can imagine your "no, thank you"s being an invitation for a fight for the next 40 years of your life???). No means no is not just a saying, it has to mean something.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.