The humans of Japan share their islands with hundreds of species of yokai, paranormal beings variously described as goblins, demons, and monsters.
Like the gigantic terai oni, who stands upside down to wash his hands in rivers, and is only dangerous if he happens to step on you, or if you're caught in the stream when he takes out his giant penis to urinate.
Multiple-tentacled yokai are eager to invade every orifice of any woman or man who falls into their grasp.
There's a whole genre of pornography, shokushu goukan, dedicated to depicting the disgust, pain, and pleasure of the victim.
They probably believe that humans can breathe underwater.
A 2007 film adaptation starred Eiji Wentz, who is the subject of gay rumors.
See also: Gay Manga of Japan; Japanese Tentacle Porn