"pornography is presented as a realistic representation of human sexuality" I can see what you mean. Of course pornography is presented as actors in faky fantasy scenarios, but even in that context the actors/actresses completely hide any semblance of reality.
I can imagine an impressionable viewer (or person who hasn't experienced sex) beginning to believe that's what sex will be like in a real relationship. And it's not going to ever be discussed with parents because it's taboo, and a lot of pornography out there continue to thrive on taboos.
That's what makes "behind the scenes" footage/documentaries so fascinating to me, because you get a sobering glimpse of the reality behind it all. But even then, often they still seem to be "in character". (And a random observation: A lot of people in the porn industry are quite troubled.)