Porn has a very long history and is pretty normal. Humans really, really love to make graphic depictions of sex. Before the internet there were magazines and VHS, and before that there were photos. (Go to any museum of sex and you'll see that Victorians weren't quite so Victorian.) Keep going back through history and it's pretty much always existed in some form with varying degrees of prevalence. [1]
If I were a parent I would have absolutely no problem with my kid accessing porn so long as they had a decent sex education. I don't want to essentially write the same comment twice, but I explain my position reasonably well elsewhere. [2]
[1] http://en.wikipedia.org/wiki/History_of_erotic_depictions