Hentai was a type of pornography.
In 3174, the Habitat had Hentai sections that tended to be only be used at night. They included Ecchi rooms, which were pressurised for nonoxygenation metabolic clientele. (PROSE: Burning Heart)
Titanian Queen Flight Technician Harvey fell asleep watching a handcuffed "Hentai girl" on a modified seesaw via a bootleg comix cube. (PROSE: Ship of Fools)
Richard Tarquin Prekodravac once insulted a high ranking member of the Kan Sun Hentai City Yakuza. (PROSE: Ship of Fools)
The Praxis IV Practical Hentai Effects houses created an animatrontic Shoggoth with tentacles the size of an elephant for one of Jason Kane's xenopornographic productions. (PROSE: Jason and the Pirates)
Behind the scenes
Thus far, every story that mentions Hentai in the Doctor Who universe has been written by Dave Stone.