I tried to protect my children from exposure to inappropriate material. Tried for years. And failed.
So I don't know what the answer is for the Internet.
In my opinion, the best solution for that is called cuties.
If my kidds even see a couple kissing on TV, they usually go like eeh-yuh. By the time they change their opinions about that will most probably be around the time they are old enough to see whatever they want anyways. (I think that's about 14 years old, by the way). I've never tryed to impose on them any content limits and things have turned out quite alright.