Here's an interesting article, submitted by a reader. The cases this doctor describes certainly fit the pattern of neurochemical addiction. This means porn may not be as harmless as we humans wish it were.
I include this link to suggest that knowledge of the brain chemistry of sex explains troubles like porn addiction. I do not include it to moralize, or make anyone feel guilty. Like any addiction, this one can be overcome, and a clearer knowledge about how our neurochemical mating program works can make it easier to regain control.