It won't work on a technical level, Quentin.
As an example, consider the "great firewall of China" which blocks YouTube (among other things). A friend told me within half an hour of landing in China his teens were happily watching YouTube using a VPN they'd installed to get round their school's net nanny. If teens can get round the Great firewall of China that easily, I suspect they'll get round whatever half arsed solution the government contracts a Pornhub subsidiary to put in place.
Also, Prawn's point applies. Your internet security may be tight, but what happens in friends' houses? The dad is a pornhound, the kids work out his PIN, sneak downstairs in the night during a sleepover and watch dad's laptop.
It's horrible but that's the truth of the situation.
I'm trying to approach it with my son by age appropriate talk, talk and more talk. Consent. Keep stressing that it's meant to be fun for both parties. That you should only ever be having sex with someone you like and respect. That friends will, over the next few years, show you stuff in their phones that is upsetting and disturbing, but that real sex isn't like that.