This is only the start
I'm amazed that the public are now receptive of this idea to the extent that it will certainly go through without large demonstrations.
Of course in itself it's almost defensible - I mean who can argue against the idea of protecting children from harm? Note though that the government frequently confuse the situation by inter-mixing the terms kiddie-porn and porn when presenting their case. Kiddie-porn is bad - BUT this block will have ZERO effect on it, so the term should not even be used.
But let's assume that Cameron's threats are real - what happens next?
Well of course some porn will continue to get through the filters - there are so many sites where porn makes a percentage of the available material, but the site as a whole is not a porn site - Tumblr springs to mind. Is Cameron then going to live up to his threat to introduce legislation, presumably making it an offence to allow porn material through to a filtered account? If so then the ISP's are going to have to blockade far far more sites that are mostly non-porn in order to ensure that NO porn ever gets through - they are also going to have to err on the side of caution, meaning that they are likely to block sites first and ask questions later.
The next stage of course, is when the government decides to use the same mechanism to include other material - first up will be "terrorist" sites and banned organisations.. who will be able to argue with that? Then this list will inevitably grow - groups such as EDL are on the edge of being banned.
Then of course any sites that are deemed offensive will follow: the law is kind of vague here already on what material is illegal and what isn't, but from what I've observed recently, if a sufficient number of people are offended, that's enough for the judge..... goodbye to jokes about disasters, anti-religious views, ANYTHING on sex.....
ALL of this will be sold to the public on the basis of preventing them from breaking ALREADY existing laws. Of course the blocks are going to have to be oh so much wider than that in scope.
Then someone is going to suggest that instead of using a white-list, we use a black-list.
Of course while all of the above is occurring, other events will occur.
It will slowly become socially unacceptable and legally awkward to opt out. People's status will be revealed in court proceedings, and in the absence of other evidence prosecutions will still push the case that this man opted out of the porn block, therefore he is more likely to be a rapist.
Eventually only a small minority will dare to opt-out, at which point the government will probably decide to remove the option completely.
George Orwell picks up from this point onwards....