I don't think that puritanism is the problem here. Adult content, especially users made one, is notoriously hard and risky to moderate. Just one underage model

I don't think that puritanism is the problem here. Adult content, especially users made one, is notoriously hard and risky to moderate. Just one underage model on your site might be enough to bring everything down, stain your reputation for a long time and under right consequences, even land you in jail. I guess payment processors refuse to cater such type of business for precisely the same reason as nobody wants headlines such as "Payment Processor X is used to fund child porn".