What's stopping a large, profitable company like Valve from starting its own payment processor? Surely the technology part of it can't be an impossible hurdle.
The reason mainstream websites don't use CC Bill and it is used almost exclusively for porn, is because they charge a lot more than Stripe and the more mainstream payment processors do. There isn't really a ban on this kind of material, provided the platform hosting it is willing to use alternative processors, so much as a price increase.
Even with Pornhub and OnlyFans debacles, they never hosted content that can't be found elsewhere on sites that allow you to pay with Visa and Mastercard. The reason those platforms were targeted was never the content itself, but non-compliance with rules that professional studios have always had to abide by requiring they keep copies of government-issued ID of all performers and provide those to any viewer who asks for it, in order to be able to prove they aren't accidentally hosting content with children or otherwise non-consenting performers.
Ultimately, if a website wants to host content with a guy taking a shit on his twin underage daughters, they can do that, and you can pay for it with Visa and Mastercard, as long as they use something like CC Bill for processing and they keep adequate records enabling them to prove the "underage" characters and not actually played by underage performers, and the people involved aren't actually related. Or, maybe more precisely, you can have real twins in a scene together, but you're then limited by byzantine country-by-country laws I have no personal knowledge of regarding what they are and are not allowed to do together that counts as sex.
The Internet is where nuance goes to die, so this all gets distilled down to "Visa and Mastercard don't allow you to buy porn" by the time most people find out about any of it.