This is a great article on the destructive nature of pornography.
Pornography, by its very nature, leads to insatiability. One picture, stored in the memory, will never be enough to continue arousing a man. God, after all, designed the man and the woman to be satisfied not with a single sex act but with an ongoing appetite for each other, for the unitive and procreative union of flesh to flesh and soul to soul. One seeking the mystery outside of this covenantal union will never find what he is looking for. He will never find an image naked enough to satisfy him.