If that's the case, then you have to 1) reduce requirements on the payment processors to do due diligence and/or 2) push handling of risks associated with payments elsewhere.
Steam has a specific return policy that it applies everywhere, the games don't show up on card statements, and Steam will lock your entire account for charging back.
If there's a secret epidemic of people making new accounts specifically to buy and chargeback on adult games, the solution is for the processor to pressure Steam to take measures to reduce fradulent chargebacks (e.g. through minimum account age requirements for adult games), not to censor the platform.
Secondly, I was talking about categorically changing how we treat MC/Visa. If you are going to change their class of operation, something has to give. And it gets complicated.