The rules of Itch are clear
I think it is a little bit more complicated than that. Obviously you can pay for nsfw content with something like paypal, otherwise you would not be able to pay on Steam for that type of content. Or am I misinformed about that? Same with Patreon. Lot of smut there too. Or can't US citizens pay for their cartoon smut on patreon?
Also, paypal does not even ban nsfw content. It bans cigarettes though. But you have to have approval according to their tos. And I guess they would deny that approval depending on your location and if you are a business with valid tax information or just an amateur. But if given you could sell actual real life porn downloads/streams with friggin paypal. Yes, there are sites that do sell that and you do can pay with paypal there. So this whole cartoon smut situation is ridicoulous. Itch should business up and do it like the grown ups or at least offer an optional way of handling this, instead of driving business to Patreon, Steam, Nutaku and others.
The gumroad thing, well, they disallow a lot of content, like homemade food. But you also can sell a lot more stuff there.