Tentacles at least go back to Hokusai and probably earlier, though. (Notably unlike your average modern tentacle scene that one's implied to be a consensual encounter; make what you will of that.)
Always was under the impression Japan's plethora of weird fetishes (as in above and beyond the usual) had more to do with the serious and constant social and cultural stresses they live under, and have for several hundred years. People tend to get a bit odd under such pressures.
Wonky censorship laws and no less goofy loopholes in them are a different matter entirely, of course.