The internet while being in a sense "public" actually requires "Private" things to be accessed... IE. a computer or web surfing device.. an subscription to an ISP.. etc... as an adult I have not found myself being surprised by "porn" while browsing in any time I can remember... now if I look for it thats a different story...
the internet is very different than taking a walk in the public square and finding a 2 people in the act of sex ... or some guy with his unit swinging in the wind...
as far as an earlier question:
So do you agree with the statement that “websites clearly announce what content they have inside”?
obviously this is not the case as there are "campers and such that take advantage of misspellings and such... and who in the hell would have known what "goatse" was (warning do not google "goatse").... but any good filtering software does not rely on the websites name or even voluntary ratings... most use search crawlers to determine content ... as a parent get good filtering software ...
It's goatse.cx, and the probably intentional similarity to the words "goat sex" gave me pause the first time I saw the link. Note that it does not actually depict goat sex, but don't click on anything related to it anyway.