You obviously didn’t know how it works if I had to explain it was already possible.

If you read my comment properly, you'll see that I wrote: "I know TLS termination and interception and recertifying with custom certificates is a thing"

And it isn’t “madness"

Yes it is. TLS interception should never be normalized because it breaks the chain of trust upon which TLS is based. It can be useful in some situations, like the fortigate firewall where you control the certificate, but ISPs nor the government should be trusted to wield this power over virtually the whole country. It is a very slippery slope.

I am not aware of any mobile device that prevents you installing a new root CA.

On Android, apps can't install their own root CA. The user has to manually download it, then jump through a bunch of hoops and deeply nested menus to install it and in the process ignore all the scary warnings that their communication may be intercepted if they install and trust this certificate, and (at least on Pixel phones) they get a permanent warning in their notification tray that someone may be eavesdropping on them. Which is correct.

It is a vastly better option than onerously demanding adults provide their identity to random and potentially adult themed websites where they could be victims of identity theft or extortion

I'm strongly against government mandated age gates myself, but you're objecting for the wrong reasons. You're not providing your identity to the adult website. You're providing it to the third party identity verifier, who then certifies to the adult website that you are an adult without passing on your actual identity. Keep this in mind when you're arguing against it, because pro-age-gater puritans can use it to undermine your argument.

I object to it first and foremost on principle. I shouldn't have to request permission from a third party or the government to do perfectly normal legal adult things in the privacy of my own home.

Secondly, there is still a privacy problem at the "identity verifier". They may swear up and down that they do not store my identity data, but there is no way to prove that one way or another so I cannot trust that my data can't be leaked through them.

Thirdly, when viewing adult content, I don't want there to be any association between my real identity and the adult content whatsoever, even through a third party, and I don't want there to be anything that uniquely identifies me.

Finally, I object to the (re)demonization of all things sexual in our societies. We seem to be backsliding into puritanism under the guise of protecting the children, while we're doing nothing to protect them from real actually harmful online things that are damaging the younger generations beyond repair.

I have a Gen Z stepson, and all the ways in which he is fucked up by the online world (no attention span, permanent online-ness, no real world friends, always seeking instant gratification, unrealistic expectations about life, an overly materialistic worldview, plenty of manosphere bullshit, ... ) have precious little do do with viewing porn.