So far as I can tell, it's a clear line.
I do not come down very hard against violence or explicit content in the media, so long as adults are given the ability to control it (now if only they would).
Looking back on media that I consumed, even before high school, I'm amazed now at how much graphic content and adult themes I was exposed to. I will pick up a book that I read in the 6th grade and think, "I corrupted my young mind!"
In fact, I probably had zero understanding of the gravity of such things when I saw it.
But was what I read/heard/watched/played pornographic? I don't think so.
The line between the pornographic and the merely dramatic is partially a function of the author and partially a subjective quality in the consumer.
The same work can be either merely dramatic or pornographic for two individuals.
I believe the dividing line is drawn based upon the degree of subjective distance allowed between the consumer and the character.
In good storytelling, I fully expect conflicts to arise that are violent, problematic, etc. What I do not expect is an invitation by the author to take unreflective pleasure in evil acts.
As I said, the same artwork can be experienced in either way by different consumers. I do not doubt that some individuals play Grand Theft Auto perversely. But mostly, the thrills of the game do not feel substantially different than children playing "cops and robbers".
God of War 3, on the other hand, deliberately requires you, the player, to enjoy the very physical feeling of murder.
What about sex?
I believe it's a mistake to put sex on the same level as violence. It's worse. Perhaps that is counter-intuitive to people in my age group. "Make love, not war," etc. More Puritanical prudery from your local traditionalist Catholic. Yet there is a very simple reason why.
I can watch one man shoot another man without participating in the shooting. I can even pretend to shoot a man in the context of a game and not participate in it. Violence, whatever its inherent evil, does not by spectatorship or pretend compel complicity.
Sexually explicit content, however, does. That's partly cultural; but let's not fool ourselves: biology is a major player. It doesn't matter whether a novel, a film, or a game creates any level of distance between the actors and the consumer. Any recreation of the sounds, sights, and sensations of sex elicits immediate responses from the body. It is impossible to enjoy the "dramatic" element of a fully explicit rape scene without becoming emotionally complicit in it, at least partly.
Some scenes are carefully manufactured to communicate horror (think of the miserable sequence following the "key party" in the Elijah Wood film, "Ice Storm"). But even they eliminate nudity so that the horror is not compromised by natural reflexes.
The recent media spat over sexual video games is probably the best advertising they have ever witnessed. But their growing popularity likely has more to do with the lingering 60's cultural narrative that "pornography" no longer exists. So sayith the legend, the label of "porn" was only ever an instrument of patriarchy to control the free flow of information and maintain stultifying control over children and women.
So now we will have an epidemic of people who are controlled not by patriarchal institutions without, but by addiction, compulsion, and insecurity within. Hail freedom. Right?