Wow. This issue certainly sparked a lot of... interesting conversations.
I'm kind of amazed to see hentai compared to child pornography. The latter is illegal because possession of child pornography is possession of incontrovertible evidence of child abuse. It's seen as corroboration in a crime because if you find such evidence (of child abuse or murder or any other felony) you're supposed to report it to the police if you're a responsible citizen. There is no way this is even in the same ballpark as having cartoon porn.
The Japanese are certainly not alone in weird fetishes or cartoon porn, either. If you're inclined to look around you can find a great deal of very American cartoon porn depicting all kinds of atrocities visited on Disney characters. The Japanese have their own spin with the tentacles and machines and whatnot, but Cthulhu didn't come from Japan, and Cthulhu sex is its own sub-demographic of porn that you can find all over the internet and in many cases in print. I would also say just personally that there is nothing I could see in hentai that wouldn't disturb me more than some of the things that real people do with their bodies (the disturbing large number of people who have died of ruptured colons from having sex with horses, for instance -- and let's not even get into German scat fetishes) -- because it isn't real. Though I should say this is largely a theory on my part, because I haven't seen La Blue Girl and don't ever intend to. Still -- not real < real.
Bottom line, people of all cultures are capable of some really extravagantly wrong-in-the-head fetishes in pursuit of the forbidden and the dark side of sex. The Japanese don't have a corner on the market by a long shot.