You answered it yourself. Pedos do use some serious mental gymnastics to rationalize their moves. They start with "oh it's just a drawing, I'm not hurting kids by jerking off to a cartoon girl... right?"... and of course they do turn to real kids once they get burnt out.
Most pedos also have a fundamental issue of rationalizing and controling sexual attraction with the simple notion that you can't fuck a child that hasn't even had her menarche. Like that in and of itself is a telltale sign of illness. Also, aging slowly shifts one's perspective on what's acceptable to fuck. A 14 year old boy who's just jacked off for the first time will be attracted to 14 year old girls. As that boy ages, so does his perspective shift and change to stay in line with the generation he was born in, with an +- 2 to 3 year span being acceptable. After 21 a normal person wouldn't even dare try to touch anything under 18. Well, pedos don't have that. Their brain stays stuck in the position of the 14 year old forever, never truly sexually maturing and following his generation.
Add to that some severe mental illness (usually autism) and multiple fetishism (which is still a psychiatric illness, at least according to the ICD 10) and you get Goonclown.