The word hentai in Japanese denotes aberration and is commonly used to refer to cartoons and animations that portray graphic adult behavior. It is also used to refer to pornographic animation works.
Sketches dealing with sex can be found in Japan going back several centuries. One such sketch shows a naked girl with an octopus around her. The word H deriving from etchi is said to be used in Japan to refer to any explicit/sexual content or product.
Hentai publications deal with most of the common fetishes found in any pornographic industry. On casual observation, the dominant colors used are bright red, blue, and orange. The facial expressions are standardized with big eyes and an almost negligible nose.
Searching for hentai in a search engine picture search would show what hentai cartoons look like. A closer look at the facial expressions would show cartoon characters engaged in sex with looks of discomfort, pain, awe, and unease. This appears to be the most common set of expression for females.
If hentai portrayal was really closer to reality, one wonders if such would be the case. The expressions of discomfort and awe show a detachment with the act...