Maybe I'm daft but how would ISPs be able to control it? Solely based on domain name? What about porn intermixed with other content on sites like reddit?
I assume they use some kind of regex to do this, and it works.
Correct. The first iterative step would be to simply block the big obvious domains and sources escalating the arms race. It will never be possible to keep 100% of porn out of your kids' hands, but just like alcohol, cigarettes, and getting behind a car we can setup a better starting point than "Do you promise you're older than 18/19/21? YES/NO"
It's a bit absurd that providing alcohol to minor is punishable by a fine, but we give the internet/porn industry a free pass. Also if you were to hand out porn to a minor you'd probably be hit with some kind of charge (inviting a minor to some sexuality), but the Porn industry is handing it out en masse without any repercussion