Itch.io are welcome to sell whatever they wish at their discretion, as is their prerogative. Personally, I don't purchase NSFW (porn style) games, but it doesn't matter if I like them or not: adults should still be able to purchase them without judgement if they see fit - to enjoy them in the privacy of their own home without having to resort into going to some sordid and potentially dangerous corner of the internet to obtain something perfectly legal, a work of pure fiction at that. Someone, somewhere, should be allowed to sell these titles. Having payment processors push everyone around is a net negative for all us consumers. I think it would do the opposite of aid us in the long run, in fact.
Others have mentioned problems with parenting and circumventing age verification. Having made a purchase, Itch.io is a DRM-free store. Using this site or others like it, such as GOG, grant parents complete control already. All they must do is vet, purchase, and install appropriate games, then remove internet access. The procedure is no different than when games were distributed physically. Parents just need to take accountability for their own actions. Granting young children unfettered access to the internet is a very foolish thing to do that, in my opinion, parents should be much more considerate over.
Requesting governments or corporations to do this instead of parents having the sense and individual liberty to do so just brings what we are already seeing: widespread censorship. These agencies are being allowed to control what we see, what we purchase; what we think.
I don't do anything immoral or illegal, but I don't really want other people knowing what I do in private. We all deserve that right to privacy. Let me know what you think!