The problem here is that we are not talking about sex or intercourse, we are talking about porn. Porn, almost always, objectifies women, promotes sexual violence, and fails to show that which is beautiful about sex, passion. 90%+ of all porn is meant to be dirty and nasty, how is that good for children? Maybe I'm alone here, but watching a woman get slapped and called a whore over and over isn't what I would call a beautiful sight.
On the other hand, neither is watching someones head get filled with bullets. The difference? Violence doesn't discriminate, it doesn't objectify, nor is it promoted by it's vary nature. The act of watching something violent does not make people WANT to partake in violence, with a few possible crazy exceptions. Porn on the other hand is meant to do just that, to make you want to have sex as it drives your hormones. Is there something wrong with that? Well, it is best for children NOT to want to have sex anymore than their natural hormones will persuade, but I also don't think it is the end of the world. The problem arises when they start to objectify women and look at them as less than human, which I'm sure most of us can remember having childhood friend that did just that. That is wrong and is a horrible by product of some parts of our society and should be guarded against.