I'd be interesting to know if Valve is big enough to start their own payment system. Yea, I know it would be hard but their customers have libraries of games in their system and Valve has lots of good will. Valve could also offer discounts ($X off if you use ValvePay). It would take years. They'd have to drop the adult games now, start ValvePay, promote it until the majority of their customers used it. Then put the games back and tell Visa and MC they can eff-off.
Simpler: why can't I buy nsfw games with a regular dumb bank transaction (SEPA €) in my case?
If MasterCard/Visa don't want these transactions, stupid, their loss. But at least let me use a payment method that works & doesn't have these morale restrictions?
Or even a Plausible Deniability system - you can't buy these games with currency, you have to use tokens. Here's a token store where you can buy tokens with your currency.