An argument I tend to hear is risk management, since the adult industries deal with a higher level of fraud. (And shameful refunds, perhaps somewhat understandably.) I think what we are likely reacting to is a legitimate desire to curb this sort of fraud that has evolved, over time, into a moral panic because inconveniently legal vices correlate strongly with that fraud.
The solution cannot be to turn Visa/Mastercard into the morality police. Or any payment processor, really. That is not their job, and they are ill equipped to perform it. Hard agree that access to payment processing should be based on legality of the sale and nothing else. If Visa/Mastercard want to then *measure* a business's overall fraud level as it happens in reality, and then adjust their rates accordingly, they can still do that in a fair manner. In other words, the riskier businesses deal with higher fees or something, but we aren't trying to define whether furry art is somehow porn or some other nonsense in the crossfire. Separate the streams please.
