Scott Adams pointed out the problem fourteen years ago

Ed Vaizey has announced plans for UK ISPs to block porn sites, with them being unblocked only for those who get themselves put on an ‘opt-in’ list. This is apparently to protect children.
Now, I’m no fan of porn or the porn industry, but nor am I a fan of other people deciding what I can or cannot see or do, based on the presumed inabilty of others to control their children (a presumption that many parents of my acquaintance emphatically do not share). If you don’t want your kids looking at porn, then don’t give them an internet connection – though I must say that having grown up pre-net, I don’t remember pornography being particularly scarce or difficult to obtain.
And this change will prevent many people from accessing useful information – it is absolutely certain that, for example, many LGBT sites will be hidden behind this Great Firewall. What it *won’t* do, however, is prevent horny teenagers from viewing pornographic material. Just off the top of my head, some obvious ways round it:

Pretend to be your parents (or have an adult-sounding friend do so) and get put on the opt-in list
Share material via P2P networks
Private FTP sites
Anonymising proxies
Usenet
Photo-sharing sites like Flickr and video-sharing sites like YouTube
Get a Gmail account, share the password with a few dozen friends, and have any of them who are outside the firewall or who have other access to this material upload the files to that.
Share files through IM networks
Share files through ‘sneakernet’ – using USB sticks etc
TOR
Hop on a neighbour’s unprotected wireless network and use their connection

…and so ad infinitum.

Quite simply, no technical solution will work – social problems need social solutions. All this will do is make life slightly more difficult for a whole bunch of people, cause at least some people to lose their jobs (the ‘opt-in’ lists *will* get leaked, and those on them *will* be treated as suspicious), make further attempts at censorship of the web easier, and further encourage a tendency in the government to try to control every aspect of everyone’s lives.
There’s a simple way to tell if I want a piece of data – I send a HTTP GET request for it. It *should not* be necessary for me to specifically tell my ISP, separately, that I might want that data – sending the request is prima facie evidence that I want that data.
Prohibition does not work, has never worked, will never work, *CANNOT* ever work. Anyone with the most rudimentary knowledge of cybernetics, of human systems, of the things that politicians of all people should understand *KNOWS* this. Prohibition with added computer is still just prohibition – computers do not magically make the impossible possible. This scheme will damage free speech, prevent some LGBT teenagers from finding useful information, and have about as much effect on the ability of libidinous teenagers to find pornographic material as just asking them not to look would.
Stupid, stupid, stupid, stupid, stupid.