Mastercard don't care you want porn, or games, or whatever. Neither does VISA. They like money. They want money and want people to move their money so they can siphon off some of it for their own pockets. Almost nobody is going to avoid using a bank because their card provider let some other people buy rude games on steam.
The payment processors don't care. They want you to send money through them so they can take their cut.
Steam doesn't care. The people making the games don't care. They all just want to sell stuff.
The only thing that impacts this really is chargebacks, which iiuc are much more common with adult stuff.
But payment processors can't guarantee what mastercard or visa will do, and players like steam (and they're huge, this is not about tiny store issues) can't guarantee what payment processors will do and given the potential downside - blocking all sales - people need to be careful.
While I can see how these situations come up, it's also absolutely insane as an end result because I just want to give *my money* to someone else. I've ended up using crypto before for buying things, not for ideological reasons, but purely because I could buy them and then give them to someone else for the "flagged as risky" goods/services because I couldn't pay for things using my money and my card.