I'm not sure I agree wth this totaly, I agree with the desensitizing effect ( seen one porn movie seen them all), I also agree with the baby steps analogy. However to take what you have said at face value, would mean every one who was addicted to porn would follow a natural path to rape/murder. This is clearly not what happens.
Porn, like all limbic triggers is basically addictive. All things that are addictive have some degree of desensitizing effect. Once the subject is desensitized they need increasingly additional external stimuli. People go from porn to rape/killing... not from nothing to rape killing as katja assumed. There is always a progression to human depravity, always babysteps, always getting away with a little more.