If it was to stop kids you’d have these sites set a header or similar and allow parents to use their existing parental controls to enforce it.

Yes exactly. That would simply solve it.

In Holland there's even ISPs that filter porn and stuff, like https://kliksafe.nl . They're used by ultra-religious conservative communities (calvinists). Even adults use it there.

I view this as a much better solution. The people that want it can do their blocking and the rest of us aren't bothered with verifications and stuff.

Personally I belong to the sex-positive movement which thinks diametrically opposite about such matters :)