It's called "cuckolding", it's a fairly common fetish, but for some reason I've always associated it more with straight, married men rather than gay men. But the point is, you're not alone, there are tons of guys out there who get off on being cheated on.
You need to be able to discuss your sexual fantasies with your boyfriend, and vice versa. Don't pressure him to actually do it, just bring it up that it's a fantasy that really yanks your crank, while at the same time making sure that you're doing everything you can to fulfill whatever his sexual fantasies are. Maybe at some point he will come around, maybe not, but you'll never know if you don't throw it out there. Cuckolding is kind of high-stakes sexual kink, with a lot of landmines for the relationship if you're both not fully on-board and prepared. For instance, right now you enjoy the fantasy, but you may not be so crazy about the reality. There have been guys who have hounded their wives into getting fucked by another guy, and then freaked out in the moment when it actually starts happening in front of their eyes. I've also heard of cases where the cuckold (you) has decided that he no longer wants his wife to get fucked by other guys, only to have the formerly-unwilling wife refuse to give up her newfound sexual freedom. So there is a lot of potential danger inherent in this fetish, most of which needs to be hashed out in detail before you actually start to make it happen.
BTW, he should at least be willing to make a few overtures towards fulfilling your cuckold fantasy...say, talking about getting fucked by another guy while you both masturbate, maybe engage in a little low-stakes internet flirting or phone sex with some guys and throw it in your face, that kind of thing. Who knows, it could progress from there into the real thing. Good luck!
