Porn is not unhealthy. In fact, I would say it is very healthy. Porn is what keeps married men from cheating. It's porn for a reason. Pure fantasy to satisfy that area of human instincts for both men and women. To deny and/or defy it is going against mother nature herself.
I think this really depends on the man.
Following what CharlieBoy stated, I know quite a few fellows who have really distorted views of what a woman should look like and how she should perform in the bedroom as a result of a steady diet of pornography.
I can't see how limiting porn is going against mother nature either, as I have yet to see a pair of breasts in such entertainment that do not deny and/or defy her.
~Sphinx
You don't need eyes to see, you need vision. ~Faithless, Reverence.