Generally it's not rare to see websites restrict their content not because its illegal, but because it's impossible to collect payment for any of your content, if some of your content is deemed unacceptable by VISA/Mastercard, etc...
And patreon marketed itself to content producers specifically as not following the paypal/stripe example.
Sage? GoCardless? Coinbase? Etsy? A platform that pays out specifically for nsfw content (like pornhub)? Hatreon, as many have mentioned?
If no available alternatives suit, and there is a big demand for something different, someone passionate should go build it and get rich! And if there isn't a big demand for it, so be it! Not everyone gets to have a liveable salary from writing furry fanfic porn, c'est la vie. The entitlement in the letter really bothers me.
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.
If no available alternatives suit, and there is a big demand for something different, someone passionate should go build it and get rich! And if there isn't a big demand for it, so be it! Not everyone gets to have a liveable salary from writing furry fanfic porn, c'est la vie. The entitlement in the letter really bothers me.