We have laws that prevent minors from buying alcohol and cigarettes, we don't expect parents to police this. Why should access to internet porn be different?
Well actually parents do police this. It is illegal for kids to purchase alcohol and cigarettes from a shop, that is all that the law actually stipulates - it's not illegal for children to partake of alcohol or cigarettes in the home.
Parents are expected to ensure that their children do not partake of alcohol or cigarettes when they are in their own home.
So equally (to my mind) parents should ensure that their children do not have access to internet porn when they are in their own home.
(Very simplistic I know)