After WWII US occupying leader imposed US legal standards on Japanese judicial system, our society became more flexible with it and adapted, they sort of froze it in time. It's not about Americans being prudes, it's about that time in history.
How the hell you figure that?California is as west as it gets,actually its the land of the fruits and nuts lol..Porn Central is California,how you figure prudes?
Which they only do because Western (especially American) prudes shamed them into it, and the Japanese went along with it to seem as "civilized" as the West. So it's actually our fault!