Depraved videos of people being brutally murdered, see "3guys1hammer" for example, are not illegal and they depict the most heinous of acts. I am sure a depraved sociopath or two has masturbated to such videos before as well. Yet, while these videos are legal and a certain segment of the population with extremely violent tendencies may experience sexual pleasure from them, having them legal has not increased the incidence of people making murder/rape videos on a for profit basis.
The whole thing has devolved into pure insanity. People being thrown in jail for pictures of the Simpsons having sex, people being framed, the FBI posting links on sites purporting to be child porn, then storing the IP of anyone who visits that URL without verifying in any way that they were in fact referred from the site where the link was posted. So long as the underlying act is kept illegal, legalizing possession of data depicting such acts does nothing to boost crime rates. I would also imagine it would be even easier to locate and prosecute actual pedophiles if such images were legal.