Thinking about it you're right, it's just a kink. I still think rape or torture as a main storyline is extremely weird, but that's not a reason to ban them unless an informed majority wants to. In any case, it shouldn't be the choice of payment processors.
Even if an informed majority wants to ban something, it should not be illegal without actual evidence of actual harm. And all legal enterprises should be able to access the infrastructure necessary to conduct business.
