I don't really like all these anti-porn laws. If kids want porn, there are too many leaky buckets they can drink from. Hell, they could just messages pics and videos to each other.
That said, if we are forced to do strong verification, the best I can think of is some sort of mix of the ideas we use for certificate authorities and oauth.
Certificate authorities are really just trusted identity providers. In my solution, you would choose from a list of trusted identity providers. You provide them with all the private information necessary for them to validate your identity. From there a third party can validate information about you with your permission.
The way this workflow would work is similar to oauth workflows people are familiar with for Google, Facebook, and other single sign-on solutions. You go to a adult site, select your provider from a list of trusted identity providers, the adult site redirects you to the provider site, you log in and give the adult site the privilege to verify you are over 18. The browser redirects to the adult site. The adult site would get nothing else about you besides what identify provider you use and if you are over 18.
Now ultimately, you have to give your private details to someone but at least you don't have to give it to everyone. Unfortunately, your provider could potentially keep track of what sites you are allowing to verify your information. We would need strict laws on these providers on what records they are allowed to keep.