It would probably also be illegal for the government to mandate a paywall.
The issue is not that age gates are illegal, but that the government forcing people to use age gates is illegal.
The issue is not that age gates are illegal, but that the government forcing people to use age gates is illegal.
The government can ban the sale of those things to minors, generally. So the category of porn sites that require a credit card and pay gate the content might be regulateable.
But that's not how places like pornhub or xvideos operate