Visa does not allow that. The rules are worded as "If the merchant has _any relation to_ adult content, banned", not "if the merchant accepts visa payment for adult content".

It depends on the adult content.

Actual porn OK. Onlyfans OK.

But fiction, for some reason, is not.

That's not true, Visa/Mastercard also pressured Onlyfans a few years ago, and Fansly just banned content recently because of them too, so they definitely also pressure them