Incest and rape, eh?
I enjoy stories too, and occasionally wander into that forbidden area...
Generally speaking, the incest stories are indeed better than average. The sex scenes are, I don't know, easier. The people already know each other, secretly want each other, and then do those forbidden things. It's just exciting. Does that mean I would bang my (hypothetical) sister? Hell no. Real incest is probably icky and awkward, but that's pretty much irrelevant.
As for the rape stories: those are mostly about power and control (just like rape in real life). The fact that some of those stories excite me or your boyfriend doesn't mean we want to rape women or would even consider hurting them.
Sex stories aren't the problem, even if they're about rape/incest. The thing that worries me about your post is that your husband seems to be pushing you away. He might be ashamed about those stories, or about the fact that he masturbates so often. Perhaps he's addicted to masturbation and sex stories. Or perhaps he and his hormones simply want more sex.