I would fully back the 'opt in' suggestion for porn.
Would that block, or not block, a search like
encrypted.google.com/#q=anal+sex+site:.mumsnet.com
Block it, and everyone who uses Mumsnet (quite a few of whom are parents of young children) would need to opt in.
Don't block it, and your claim to be filtering material harmful for children looks a bit threadbare.
What about
www.nhs.uk/chq/Pages/3050.aspx?CategoryID=118
Do you think that should be blocked? Blocking pages on www.nhs.uk is unlikely to end well, and it's unlikely many people would not opt out of that once the consequences became apparent. Not blocking detailed descriptions of anal sex is likely to get the Daily Mail upset.
What about
www.ebay.co.uk/itm/Luxury-Stainless-Steel-Adjustable-Locking-Female-Chastity-Belt/281148911330
A blocking system that manages to keep up with every sale posted on EBay is going to be a bit hard to do, isn't it? Probably easier to just block EBay. How many people are going to be happy with that? No naughty words on that web page, either.
And so on, and so on, and so on.
The chances of being able to produce a filtering solution that (a) "keeps children safe" and (b) doesn't piss adults off enough that they'll turn it off are approximately zero. The idea of doing it at the network level is just laughable: houses contain multiple computers, with multiple users, and the blocking in the network cannot tell them apart. It's political posturing, and can't (in general) work.