Paypal doesn't process such payments.

Mastercard have a huge list of arbitrary rules.

Visa and Mastercard charge significantly extra to process adult payments.

Ah thanks for the clarification. Do you know under what pretense they charge the higher rates? I can imagine there is more fraud for adult payments, but would they also charge higher for gambling or other fraud prone payments in that case?

One story I've heard is that adult content payments have a very high rate of chargeback, claiming it was fraud after the fact as a way of saving face when someone else (like a spouse) sees it in their transaction history.

But as you say, adult content isn't the only high-risk business but payment processors seem to care much more about it than others.

My guess is that it's just exactly what it looks like - a squeamishness to be associated with activities that culture targets as "non-respectable". "If people pay for porn with Paypal, then they'll think of Paypal as a porn payment provider and not want to use Paypal to pay for normal things".

Does stripe do the same too?