At the risk of tipping my hand sufficiently to be banned for my horrible potential depravity, this matter has often crossed my mind as well. I'm into Japanese Anime, and I'm afraid I may have become somewhat infected by the influence of the Lolita fixation influence of prominent in Japanese culture that often shows up in their sexy cartoons.
I have no intent to sidetrack children (or even teens) in real life - it's just not honorable. However, I have to wonder, should my computer be impounded, the cache plumbed by a bit-recovery agent, and racy young anime heroines in various states of undress and/or violation recovered, would I be held accountable? Is it genuinely pedophilia to have thoughts of arousal, which will not be acted upon, directed at arguably ageless drawings?
Granted, Bart and Lisa might be a different story. They're really quite obviously prepubescent, where your average anime heroine who the story claims may be 10-years-old is drawn with a body that would make a college coed jealous. But you have to wonder if John McEwan was getting aroused by lewd Simpson imagery or just thought they were funny.