Personally I think they should not be able to decide such things. A payment provider should be forced to accept all legitimate customers. It is absolutely unwanted for a private business (in an unrelated category no less) to be playing the role of legislator.

The problem is once a majority of payment providers (especially in this category where there's only really 2 major ones) starts blocking things, it becomes really difficult for fringe groups that are in fact perfectly legal. I understand that porn payments are a bit more risky but the providers can simply have a policy to deny chargebacks for this category and shift the risk to the customer. Similar to the way returns of sex toys are not accepted for hygiene reasons.

Recently there was a lot of news in the Netherlands about prostitutes not being able to open bank account because all banks refused them. Think of prostitution what you will but over there it's legal work and they pay their taxes. But not being able to get a bank account gets them into very difficult situations and pushes them to shady people (which was exactly what the legalisation was avoiding).

Also other platforms like fetlife have recently been forced to validate their users, leading to potentially serious consequences if this data ever becomes compromised. Many people there don't even show their face so the result is that they have to give a lot more personal info now than they would have wanted to.