Michio said:
It's not just the religion, it's the culture surrounding it. It also has nothing to do with diversity, but cultural attitudes.
Adultery of little boys and girls may be disturbing, but so is having your face ripped off by a large piece of wood, then having citric acid splashed on the area where your face used to be. How is that not offensive?
There is an indigenous people in Nepal that places pre-pubescent kids in dormitories and encourages them to sexually court each other so that when they hit puberty, they'll be "ready". If someone tried that in America, people would be screaming to the relevant people put to death for child abuse or whatever else.
And so we come to the conclusion that America's psycho-sexual development is much different from Nepal with its distinct culture. It's less offensive because people in America find it humorous, and many American's these days have a queer sense of humor (myself included.)
When a very old man on the popular animation Family Guy (I know, much different than Japanese animation,) hits on the thirteen year old chris, the writers script the dialogue to humor the audience.
When a foreign item depicting adultery with nothing real humorous (besides maybe mediocre Japanese jokes that might not make sense to some Americans,) contains what it seems like to be an 8 year old girl dousing her mouth with an older man or of the same age's sex, it is different.
It is more of the fact that the writers and fans admit that the depicted is of obscene under age; however, in norm porn, we can have girls looking as young as 15, just as long as we know they're above the age.
Besides, you must understand the political situation that Japan or Americans will face. Credibility of a country is made up by their people, or a large individual. Some foreign people look at America with disgust because of our: Obesity, laziness, and how inappropriate we Americans are; Japan politicians wouldn't necessarily like it if the stereotype was that every Japanese was an "otaku," or a pedophile, and the latter disgust that many major world powers see of this kind of adultery.
I like pedophilia art though, but I understand why it's banned.