Sorry for the "actually", but Hentai didn't exist yet as a genre. It was "shunga", that is, erotic "ukiyo-e", a popular style at that time.

Popular shunga works by Hokusai are "Two lovers" or the wrongly translated "The Dream of the Fisherman's Wife" (the original Japanese title is "female diver and octopus")

So? Go play the morality police elsewhere.