You don't owe this guy anything, and he doesn't sound like he is right for you. You don't enjoy sex, he has slept with fifty or more women. I think you're right, and sensible, to be worried about the health side of things - giving this guy a blow job not to keep him happy isn't worth the health risks in a few years time. And I am not saying this to be mean, but deep down I think you know that this isn't going to work - you both want different things in the bedroom. A relationship can't survive when the people in it are polar opposites.