I agree with Patagonia, he's not the one with the problem, you are the one with the problem with him. Yes, there is a whole other world out there called BDSM, and those fetishes are actually quite common despite how unusual it sounds. I'd recommend that you see a counselor to accept this part of his life, because odds are he has accepted this as part of who he is. And if he says he won't do any of this in real life, it will never be an issue for him, but don't deny him his fetish simply because you're afraid of it.