| Tentacle porn. It is one of the few terms that pretty much means the same in any language. Just the thought of slimy slithering rods of muscular flesh probing the deepest sexuality of innocent feminine pulchritude is enough to get a stong reaction out of just about anybody. (Do gay guys have tentacle porn? I really want to know!) It's like getting tied up and being gang-raped by a single individual.
It is useful in pornography, as the Octopus (or Squid) is the surrogate of the viewer. He can watch the victim get molested without having to compete mentally with the image of a competitor in the picture. Japanese Hentai (animated porn) uses lots of tentacles, both flesh and robotic.
Another reason for the ersatz penis use is that Japanese porn law prohibits showing genital penetration. Tentacles aren't penises, no matter how much they resemble dicks in any given drawing. No harm, no foul.
The interesting thing is that tentacle porn was not originally brought on by animation and morality law.
There is a famous piece of tentacle porn art by the Japanese painter Hokusai in his erotic (Shunga) book Kinoe-no-Komatsu, in 1814. Milk has an excellent wu on the scene depicted, "Dream of the Fisherman's Wife".
The octopus or squid is depicted as giving the woman oral sex, only using the tentacles (as far as we can see) to caress the woman's body. The use of tentacles as penises may be a recent development that was truly brought on by morality law.
The part that gets me is that that an octopus mouth is a beak, used for tearing its prey. (A danger Swankivy notes in the humourous prairie squid.)
I wonder if the artist knew about the beak when he created the image? It gives one pause. | 
