The same government mandates sex education in schools at a younger and younger ages with all sorts of groups trying to push their agenda on what they think is normal.
Yet as parents its not the sort of thing that you can go into much detail about without falling foul of the thought police about the moral rights and wrongs and potentially even child protection, so we have to give them some fairly generic guidance at the appropriate times, for example buying them a box of condoms at 16 for example when they probably wouldn't want to go into a shop and get their own, or talking about "more normal" activities and eluding to some of the less normal activities, then leave it for them to find out about it on their own, since they will develop their own preferences like we all have.
If they decide that the Internet is the place to learn - since this is also what is drilled into them through school and college, then its hardly surprising that they will stumble onto such sites. Finding about some of the edgier topics and realising this is not as common or just making a decision "I don't want to do that" is a lot easier there than when they get into a bad situation when they are out on their own and the other person says this is absolutely normal, and afterwards they have nobody to talk to about it.
The harder access is made for 16-18 year old's, the more that people will be there to exploit them, which is exactly what we don't want to happen.
But then again - Governments and joined up thinking rarely goes hand in hand.