That it is not cool and does not depict a normal, healthy, loving relationship between two fully and completely, freely consenting adults. That the porn star "look" is not normal - women have pubic hair and normal-sized breasts not made out of perky plastic.

If my son does decide he wants to watch porn, then we will sit down together and watch it and discuss what is going on and our emotional reaction to that - same as we do with In The Night Garden or any other film/programme, until I know he has the emotional maturity to cope with it on his own. (I honestly cannot think of any better way to turn a young man away from porn! I am anti-porn, in case you hadn't guessed.) If he watches it on his own, then I'll insist we watch it again from the beginning together. DH can join in too if he doesn't freak out about that.
Don't you worry that that will be incredibly damaging to your relationship with your son? In terms of his trust of you, his own personal identity/space etc? This coming from someone extremely anti-porn.