Why are tentacles prevalent in anime? What is the reason/motivation for the prevalence of tentacles (possibly NSFW Wikipedia link) in anime (esp. hentai)?

Did the anime subculture lead to it or does it stem from various aspects (possibly obscure) of Japanese art/culture?
Toshio Maeda, the manga artist who did Urotsukidoji, one of the first shows to use tentacles, said in an interview that he used tentacles to circumvent the Japanese censorship laws which prohibit the depiction of male genitalia. 


  Why tentacles? For some reason there were some issues about censorship about the sex scenes. When I was drawing sex scenes, sensual scenes in bed, editors always asked me not to be so extreme. Of course we couldn't depict genitals, and if you can't see their genitals, it's not so good. And why they are so fussy about that was because they were the ones who would get arrested. We are not, always we have the freedom to express ourselves – freedom of speech, I think you really respect that in America (laughs). So actually we mangaka didn't give a shit about that, but the editors, they really cared about that, so they always said, “Please, not extreme.” But when we toned ourselves down, they said, “Come on, you should do more.” So that's why I did the tentacle thing. Because tentacles are not genitals. They are parts of the body, and some creatures have more than one. Because they are not genitals, they are like hands and legs or like uh, just body parts, so it's ok to go there. And attacking ladies with tentacles is not just part of a sex scene, you can see that. It's just outdoor things. So it was kind of a flimsy excuse for drawing that. (laughs)


After that, it caught on and is memetic in anime. It's phallic and can be used to imply something sexual, cue something "naughty", or even as fan service to poke fun at the culture.