My partner likes to talk about sex with others and I've told him I don't enjoy that kind of talk and he still does it. I'll ask him to stop and he will continue to talk about it and when I ask again he goes further into detail until I'm having to be rude with him and then at that point he acts as if I'm being ridiculous and unfair. I don't know what to do.
In my years of being polyamorous, this is something I have seen a ton of variation in. Some people really enjoy talking about their sex life with their friends. Some people are incredibly shy and private about it. Some people fall in the middle of that spectrum. No one is being unreasonable or ridiculous, people just have different preferences and boundaries around this.
It sounds like you’re bringing this up in the moment, in front of his friends, which causes him to then escalate the situation. Try talking about this later, when it’s just you two, and he doesn’t feel threatened or interrupted. Let him know that it really bothers you, and ask him to stop.
At the core, this is a consent issue. If you’re uncomfortable having the details of your sex life shared with your partner’s friends, that is a boundary you have the right to set. It sounds like you set this boundary clearly and he is being dismissive of it and violating that boundary, and that is not okay. You have the right to keep details of your sex life private.
At the same time, it sounds like your partner gets a lot out of talking about sex with his friends, and that’s okay too. If possible, try coming up with a compromise that works for everyone. Talk about this in a calm atmosphere, and tell him that you really feel uncomfortable when he talks to others about his sex with you; but you don’t mind if he talks about sex in general or sex with his other partners.
That arrangement still carries some vulnerability - if he likes talking about going to a spanking workshop, and showing his friends the new leather crop he bought, they’re probably going to come to some conclusions about you and your sexual preferences. If that still isn’t working for you, that’s fine - set different boundaries instead.
If he refuses to respect this boundary of yours; if he makes it a habit of embarrassing you in front of his friends; if you don’t feel that he treats your sex together in a safe way; if he frequently acts like you’re being ridiculous when you make requests or set boundaries - leave the relationship, because those are all major red flags.