Are you assuming that a pedophile's desires just emerge, full blown and developed, out of nowhere?
I can't prove they ALWAYS do so, but it's already a known fact that humans have all sorts of brain disorders that just happen randomly. Bad genetics, disease, brain injury ... any of these things can and do cause peoples' personalities, likes/dislikes and desires to change in any number or ways. And this is to say nothing of the simple fact that every human being is different and has different likes/dislikes even if their mind is completely "normal." Many people, yes, simply are sick freaks, and do not need any sort of external stimulus to make them that way. If humans in general were that susceptible to every idea put in front of them, there'd be no such thing as an unsuccessful supermarket product, because all any company would have to do is run a single TV ad and at least half the country would be running out the door five minutes later to buy the latest variation on Oreos.