I’m Japanese and currently studying abroad in the US. Recently I was telling some classmates about onsens and how my family are enthusiasts and they were so weirded out by the concept especially when I told them that my father & brother go together the most (people are fully naked in onsens if you’re unaware). Non-sexual nudity was never a big deal to me and in Japan where bathtubs are the norm instead of showers it’s not even unusual for same-sex family members to bathe together (one at each end). Where does this prudishness come from? Especially in a country that’s known worldwide for its hyper sexuality (Hollywood, celebs, porn industry, etc.)?
Only in the official public sphere, off to the side it is very sexualized. Ads and tv shows are full of sexual references. You hit strange limits though like genitalia cannot be shown in movies (not X rated) but you can use a fake penis on men and a merkin on woman which is ok for some bizarre reason. The US forced its prudishness with genitalia on the Japanese after WW2 which is why they pixelate it. It likely has its roots in the US as a mainly evangelical Protestant settled nation vs the more Catholic and reformation Europe. Eastern cultures also have their own rules about nudity and sex but it isn’t seen as inherently sinful.