The problem is taking payment. The credit card companies don't allow most of this content and have been known to pressure payment processors to drop specific clients, with the threat of shutting down the entire company.

Visa/Mastercard have a de facto monopoly on online payments, and if they block you from taking payment online then you are out of luck.

I've known multiple people who have run adult websites or even tried to launch the adult-specific KS-clone suggested and taking payment is the constant issue.