I don't think anyone really understands what this is for....
It's NOT to stop paedophiles or rapes - it's obvious even to Cameron that it won't do that.
In fact, it's not to STOP anyone. It's rather more for STARTING things.
The output from this will be long sets of lists of forbidden words (which will be generated by some organisation using taxpayers money), and long lists of people's names who have typed those words into search engines. These lists will be put on databases and filtered to produce statistics that show how all sorts of naughty things are increasing. And that's more work for the civil servants.
These stats will then be used as evidence to provide even closer scrutiny of everyone's activity. Which means even more work for the civil servants running the system. Perhaps they will bring in a penalty system like the 'driving points' one, and have a sliding scale of fines, so that they can become self-funding.
The technology will be readily swappable with the GCHQ spy system software - indeed, maybe Cameron is proposing this because the spooks see a chance to expand into this area and have been bending his ear. They have been really short of work since the Berlin Wall came down, and have been milking terrorism for all it was worth, but I'm sure they would prefer a regular reliable stream of work like the Cold War provided, and sex looks as if it's here to stay longer that the Russians were...
In short, this is NOT a proposal to diminish a real threat. It's a proposal to generate a new civil servant work/revenue stream, and one which can't be argued against.
"Give us the money".
"What, do you mean you're in favour of rape?!!"