Yes, I've heard that the MacArthur administration had a lot to do with the whole censorship thing in Japan. I don't know whether the fascination with bondage/SM, etc. is an outgrowth of that or something more ingrained in the Japanese male psyche. Certainly the art of rope bondage (shibari, etc.) goes much further back than WWII AFAIK. I don't know about the tentacle thing--possibly an outgrowth of the whole monster genre. Many cultures seem to have a somewhat similar thing, Leda and the Swan, Zeus as a bull ravishing whatever chick that was and even the constant alien abduction/probing and 'I had Bigfoot's Baby' stories in the tabs here in the US. Despite the peculiar censorship rules, the Japanese seem to be more open and have more fun with all of this than most places.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken