As to your second point - yes, certainly teenagers can distinguish fantasy from reality. But the major difference here is that it's not portrayed as fantasy. In a movie, everybody knows that the actors get up and walk away at the end of the day - nobody is really getting shot.
On the contrary when it comes to porn, there's no doubt that the acts are really being performed, and by real people. So while a young person's ability to distinguish fantasy from reality shouldn't be underestimated in the context of movies, video games, etc -- we have something of a different beast here.
You're right that it is an uncertainty, but certainly a logical case could be constructed for at minimum further study of this idea. Unfortunately, we as a society won't do that, because we have to pretend that our teenage children aren't sexual creatures.
What's left, then, is to make best guesses based on logic. It's a poor substitute for science but in this area it's as close as we're going to get.
