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It was basically moral law set during the Meiji period that was reinforced when the West enforced their moral law on Japan after WWII.
That rape pornography isn't just in hentai. It's pervasive in their regular pornography. It's very common for Japanese porn actresses to look distressed or cry in their movies. It's most likely a dominance issue that goes a lot deeper in their culture than we could bother analyzing on an anime forum.
They also have that obsession with school girls. Similar to what was discussed with everything that seems abnormal, there seems to be some natural basis for it. Maybe they are more open about their attraction to teenage girls? My friend always says, "everyone likes teenage girls; that's why there's a law." There is definitely an evolutionary natural reason to procreate with adolescent girls, like all other animals that breed with females when they come to age. Cultural norms in the last 1000 years have arbitrarily deemed a moralistic legal age, much as it is culturally arbitrary to believe homosexuality is wrong, or marriage is important for our species.
Morals are subjective to the time and culture (coming full circle back to the topic) we live in.
