It’s the US republicans who want to do this, not me, I’m just approaching this as an interesting problem.
As for my suggested solution, the only database would be the list of sites with adult content. No new personal data would be stored about individuals.
I’m not suggesting that ISPs implement photo-ID checks, just a login with your ISP username/password (an account you already have).
I definitely agree that these types of blocking are ineffective and generally do more harm than good, but if governments are going to push for this stuff, it would be good to have a solution that doesn’t harm people’s security and privacy.