Contrary to any libertarians here, and contrary to recent Supreme Court rulings, pornography has no Constitutional rights.
If I sold gave away free beer and someone's 14 year old walked up to my stand and I gave him some...I would be liable for that and I should be held civilly and criminally responsible.
As it relates to adults, porn is an extremely harmful activity to all of the participants involved (even the businessmen who make the money off of it). That being said, so is beer ,so are cigarettes, so are lottery tickets, so is gambling, so are doughnuts. etc....
You can't prevent adults from hurting themselves except by explaining to them the dangers and hoping they make the right choice.