A very jargon-heavy article at a very legit science news site says that Russian scientists believe this is so. Not the Cthulhu part – science’s job isn’t to prove or disprove the Elder Gods’ existence – but the tentacle part.
Our study shows that there is a group of the Lophophore animals among the Bilateria — Lophotrochozoa taxon, which includes the largest variety of types of animals, — Elena Temereva says, — The lophophore is a special organ that carries tentacles. Phoronids, brachiopods and bryozoans (ectoprocts) have collectively been called lophophorate, because they have the lophophore. However, multiple molecular phylogenetics data shows that there is no united group of the Lophophore animals
You can read the whole thing, but the entire article’s like that. I’ll sum it up for you: every animal that is bilaterally symmetrical (i.e. that could be divided in more or less equal halves if sliced down the middle) comes from an early sea creature that had tentacles, some 600 million years ago.
So when you see tentacle sex in anime, it’s really just a throwback to some very ancient impulses.
(h/t Bradley J. Fikes)