Yeah, but the policy wasn't to have one "national filter". It was to have the government tell ISPs "you must have a filter". How the ISPs implement it would be

Yeah, but the policy wasn't to have one "national filter". It was to have the government tell ISPs "you must have a filter". How the ISPs implement it would be up to them. Which means they'd go buy the same crap that universities, libraries and corporations are currently using and implement that. Deciding what counts as "porn" is very difficult. Deciding what is illegal child porn is, with the odd exception, pretty easy. But for legal non-child porn, well, what about a 15-year-old girl writing erotic Harry Potter shags Dracoy Malfoy fan fiction and sharing it in the fanfic community on LiveJournal or whatever. Does that count as "porn" in the same way as commercially produced videos does? And it is trivially defeatable: one kind finds a stash of porn, puts it on USB sticks and sneakernets it to all the other teenage boys in the neighbourhood. Hell, sexting exists.