While perhaps it's a nice idea it's utterly impractical and opens the door to some very uncomfortable places. It also stops parents having to take responsibility for what their children are doing, which is the one and only real solution to the problem.
Compared to porn there really is very little child sex on the internet. However the IWF (Internet Watch Foundation) which maintains a black list of child sex web sites has a very hard time keeping up and has, at times, grossly overstepped the mark on what pages it bans and how it goes about doing that ban. It's also almost completely beyond scrutiny and control.
How on earth would it be possible to efficiently block all porn from all ISPs as any kind of a default?
It wouldn't.
And that the IWF is so random, so unthinking and so unaccountable is the reason that some ISPs refuse to co-operate with them and implement their black list.
Imagine an IWF whose workload suddenly explodes hundreds, if not thousands, of times.
Shudder.
If you hear anyone seriously talking about proposals to censor the internet their either very ill informed about how such a thing would work or are seeing beyond the possibility of censoring porn and are thinking what else they could cut and restrict.
And all so parents don't have to take responsibility of what their children are up to.