This is a unique one, to say the least!
Between 1969 and 1973, a series of three adult anime films conceived by Osamu Tezuka (best known for Astro Boy) were released. These were known as the Animerama triology and consisted of 1001 Nights, Cleopatra and Belladonna of Sadness.
All three films are considered hentai, but I don’t think anyone watches them for arousement. In fact, the first two only received a ’15’ rating from the British Board of Film Classification and the pornographic scenes would be very difficult for most people to enjoy.
Belladonna of Sadness is the only one of the three Animerama films that Tezuka was not directly involved in, so we have director Eeichi Yamamoto to thank for this, and I have to say its the most extreme, weirdest and best of the lot.