Certain materials are designed to produce sexual feelings. And I am not saying that having the thoughts is or should be a crime, though I think that having them is morally wrong.
Outlawing materials that are designed to produce pedophilic thoughts and consistently do so for some people is a way for society to say that such thoughts are wrong. I think part of being a responsible human being is controlling ones own thoughts to some ways.
Some materials aren't designed thus yet produce sexual feelings anyway. Some materials produced to generate sexual feelings produce disgust instead. This reasoning is why we have always seriously questioned the banning of anything.
Outlawing materials that are designed to produce pedophilic thoughts and consistently do so for some people is a way for society to say that such thoughts are wrong. I think part of being a responsible human being is controlling ones own thoughts to some ways.