You're forgetting that IP isn't everything. The DNS lookup starts everything, so why aren't DNS servers bogged down? OpenDNS will filter adult content at the DNS level already.
I’ve worked on devices that have done this for security applications - things like stateful intrusion detection boxes.
I'm sorry, but adult content blocking is a lot easier than intrusion detection. It doesn't even require any more than one state. Even if you're imagining some sort of complex computer vision AI that would detect porn, it wouldn't need to be doing it in real-time. You seem to be imagining a serverless internet without common content. For the most part, a user will enter a url that must be resolved by DNS, that displays roughly the same content as it did months ago.
DNS-based filtering is easily circumvented. It already is being circumvented - today.
One of the dirty little secrets of the computing and networking industries I’m here to tell you is just how well-funded and how high-tech the porn industry is. Porn outfits are the bleeding-edge customers we in the computing/networking industry don’t like to talk about. But boy oh boy, do they buy equipment by the truckload.
If the government were to put up a free WiFi network, where you could be surfing porn anonymously anywhere, the porn industry would give their eyeteeth to make sure they capitalize on that access. They already use HTTP tunneling and proxy servers to circumvent DNS and HTTP filters/loggers that companies use to keep employees from surfing porn at work on a company’s network.
That’s what makes me so certain that your method won’t work: because the porn vendors are already circumventing exactly the approach you’re talking about and there already is a cat-and-mouse game going on between companies installing DNS logger/filters and HTTP filters and the porn vendors responding by burying their porn every deeper under tunnels, redirects, proxy servers, etc.