There's a bit more nuance there, as the rules actually come from the bank issuing your merchant account, rather than some master "Visa" entity. So if you're a big player in the adult game you're going to work to find the right banks willing to issue you merchant accounts.
Those banks will also have a compliance department which will look at your content and make sure it's inline with what they're willing to allow. If you want to make adult content where consensual adults do things together there's one group of banks you can go to. If you want to make niche content with acted out violence and such you're going to find a much smaller group of banks willing to issue you merchant accounts. Or possibly no banks at all. It's interesting that the thing deciding what adult content will be easily monetizable on the internet is small merchant account issuing banks.
On the charging side I believe Stripe uses Wells Fargo, which has pretty strict rules.
Source: worked for one of the large players in the adult market a while back. Some info may be dated.
Note: one of the fun things about credit cards is that Visa and MC are issued by banks, and Amex is issued by Amex. There was a new fraud style a few years ago that amex was able to lock down pretty quick due to its centralized nature, while Visa and MC had a harder time.
Note2: I may define fun differently than you.