(1) You know (or can tell) what content your kids are exposed to on the internet. Most kids I know are quite adept at keeping what they are upto hidden from their parents and that isnt unique to the current generation.
(2) Parents have a much better sense of content, kids are exposed to on TV (where content is regulated and rated for the most part) than on the internet.
(3)Even if you are an involved parent with the ability to discuss things with your daughter, you both arent living in isolation from society. There are easily more parents and kids that don't have such a relationship than the number that do. So if boys as young as 8-10, growing up in highly unsupervised environments, are constantly exposed to extreme porn, what do you think their expectations of women are going to be?
(4)Addiction. Everyone has weaknesses. And porn sites are getting better and better at keeping people hooked. Kids are the most susceptible.
Freedom has always come at a cost. And the main cost is equality. If all kids are born equal and all families are created equal sure give them all the freedoms they want. But that is unfortunately not the world we live in today nor is it getting any more equal any time soon.