If there was a prominent online marketplace for homegrown literature, I would bet there would be corporate pushback against selling incest themed porn on it. But I don’t think there is such a marketplace so it’s a very hypothetical question.

If a book publisher was selling erotic fiction about children online, you could bet your ass they would have a hard time with payment processors.

I’m not sure you have a case with this argument.

If you read the founder of the "feminist" group she thinks that 50 shades is the equivalent of the book of Satan but about raping women.