There are a lot of assumptions being made, but the truth is, many different things happen with child porn. The porn is being created by some people, varying from relatives of the children to opportunistic strangers to gangs that are in the business of buying poor children from their families in many poor countries. Yes, those all happen. There is much more that is insidious about those images, though.
Oftentimes, according to researches and police records and court statements, a person begins looking at the images because they prefer them, and then are pressured to provide some to those with whom they trade. New images, that are not already in that particular trade circle. Many people have admitted that they had never intended to harm the children they knew - even their own children - but they had to make some new images in order to get more. So...
The statement: "There is *no* hard evidence that looking at child pornography induces people to abuse children, any more than there is hard evidence that looking at pictures of sports cars induces people to break the speed limit." is abjectly untrue. However, data of this kind is not readily available to John Q. Public, he or Jane Q. Public must dig for it.