But if you can't accept credit cards for some reason, no one would tell you to close up your business instead of accepting debit cards.
"This specific category has a high risk of chargebacks, so it's using a less convenient/safe system" does not seem like an absurd thing for a payment processor (or a storefront) to say. Services like Paypal support direct bank access for payouts, right? The systems may not be ideal, but payment processors are not so uncomfortable with them that they won't support them at all.
This doesn't really explain why payment processors couldn't restrict payments for adult material to a subsystem or charge higher transaction fees for it specifically. If this is just about risk, then... risk can be priced. It doesn't seem that complicated. You calculate the chargeback rate for the category and you price transactions for that category accordingly. Part of the financial cost of payments online goes into subsidizing risk, this is already something that we all do every time we use a credit card on a service with transaction fees.
Stripe couldn't have a different pricing tier for payments or businesses operating within specific categories?