An 1814 shunga woodcut created by renowned 19th century Japanese artist Katsushika Hokusai, "Dream of the Fisherman's Wife" features a naked woman having cunnilingus performed on her by an oversized pink octopus. Its arms are wrapped around her in a briny embrace. Coyly peeking over her head is a smaller octopus who is also participating in this cephalopod gang bang.
In October 2003, Melbourne was in an uproar over a copy of "Dream of the Fisherman's Wife" painted by an artist named David Laity. The museum displaying it was investigated for obscenity charges, which were dropped.
The woodcut itself is often cited as depicting a pearl diver, instead of a fisherman's wife. It is also said to be a rape scene. There's no explicit evidence for either interpretation just based on the woodcut. The woman doesn't look as if she's in pain, and there are no cast off clothes lying around to suggest that she was intending to do something else until she was waylaid by an octopus. Perhaps they were left out for aesthetic reasons, or perhaps the woodcut is indeed an erotic fantasy of a fisherman's wife.
There's quite a bit of text surronding the image, but I've only found one (alledged) translation for it (...the link died).
So, the next time you find yourself watching tentacle porn, keep in mind that it is not, in fact, a bizarre form of sex depravity -- it is actually based upon centuries-old artistic traditions and the anime is simply paying homage.
For the woodcut: http://www.flickr.com/photo.gne?id=49903
For the Melbourne story: http://www.buzzle.com/editorials/10-21-2003-46798.asp