bkev said:

Actually I think tentacle porn came about because of the US's post-WWII occupation of Japan. Our prudishness rubbed off on them, to the point where they passed laws saying pornographic material couldn't feature real penetration. Hence, well, tentacles. Hell, even today the tip is often blocked out -- even if just by a barely visible black bar.

Not that I know this from watching, or anything.

I think we're both right. The "creator" of tentacle porn was interviewed by Anthony Bourdain in the Parts Unknown: Tokyo episode and he pretty much said [what you said] + [what hairy_hen said].