You can do that currently. Steam already supports the exact process you described: top up your steam wallet and buy games with steam wallet balance. Actually, there are things you can only buy this way (some in-game items, not sure if it's to workaround gambling accusation or just coded so for no reason).
The issue is Visa/Mastercard/whoever is pressuring Valve isn't happy about the very existence of incest games. They don't want to be associated with incest/rape even indirectly.
If there was a prominent online marketplace for homegrown literature, I would bet there would be corporate pushback against selling incest themed porn on it. But I don’t think there is such a marketplace so it’s a very hypothetical question.
If a book publisher was selling erotic fiction about children online, you could bet your ass they would have a hard time with payment processors.
Eh yeah the Levayen Satanist book exists but I thought they were referring to a religious book from the archaic Bible ages. That'd be much more interesting to read!
The issue is Visa/Mastercard/whoever is pressuring Valve isn't happy about the very existence of incest games. They don't want to be associated with incest/rape even indirectly.