Tako to Ama (or, Dream of the Fisherman's Wife) is a beautiful woodcut illustration from 1814 by Katsushika Hokusai and the ancestor of Japanese tentacle porn. I'm not going to discuss the woman. Her particular kink may not do anything for me, but she's clearly having a good time. More power to her.
I want to talk about the octopus between her legs. 'Cos I don't know exactly what that octopus is doing, but it doesn't look anything like cephalopod sex.
For one, I don't see any evidence that the octopus is aroused. For the sake of argument, let's assume the octopus in question is a North Pacific Giant Octopus. It's about the right size and shape, and its solid red tone is pretty close to the normal colour for males in that species. But though males may look like that most of the time, everything changes when they're ready to get it on.
Octopus skin contains thousands of tiny organs called chromatophores that give them exquisite control over their colour, texture, and skin patterns. The organs are wired directly into the octopus nervous system, and let these animals instantly camouflage themselves as they move from boulder to boulder on the sea floor. Chromatophores also let these animals display exactly what's on their minds: an amorous North Pacific Giant Octopus' skin goes frilly and is overlain by a pattern of white spots on a red background.
So, he's not wearing his sexytime suit. Nor is he probing her with his sweet sweet mating arm. A male octopus does have a penis — but it's a relatively small organ that's tucked up near his head, and it doesn't actually come into contact with his mate. For that, he has a special arm called a hectycotylus.
The hectycotylus is usually a male's third right arm. It doesn't look like his other arms: in octopus its tip is flat, spoon-like, and suckerless, and a groove runs along its length from base to tip. According to Dr. James Wood, a cephalopod expert and coauthor of Octopus: The Ocean's Intelligent Invertebrate, the male uses his penis to "arm" the groove with a large and elaborate spermatophore before inserting it into a female's mantle cavity. This video of mating octopus from the Caldwell lab at the University of California, Berkeley lets you see exactly what that looks like.
As you can see, even though the octopus in Hokusai's woodcut is wrapping the woman in his arms and pulling her close to his body the way mating octopus do, his hectocotylus remains tenderly wrapped around her back. Not much spermatophore-laden probing going on there. And the woman may want to be careful about whatever probing is going on near the octopus' mouth. Their beaks are sharp, and their tongues have teeth that can drill through crab shells.