husband wants to watch me have sex with another guy.

I've been married for almost 10 years. my hubby has always been obsessed with wanting to watch me have sex with another man. So a fantasy, fine. I would act out and pretend his fantasy. but his obsession is really starting to creep me out. I told him no thats against what I believe in. To me that's cheating. but he wont stop. hes begging me and saying "please" like a little kid. I keep telling him no. he is really making me feel bad about myself and getting mad at me for saying no. it is so unfair to be treated like crap for something I dont want to do. I can't stop crying and feeling like junk because of his sick obsession. I'm so stressed out. what do I do?