Sexually explicit content
Sexually explicit content should be a no go as per site policy in both questions and answers. unless censored and answerable in a way that can comply with these policies and is handled in a scientific way
Lets first define site policy, as this should be one of the main guidelines to adhere.
Sexually Explicit Material. Accounts that use Stack Exchange to post sexually explicit or pornographic material, or links to it, will be suspended.
Considering that what people might find offensive differs per culture, I suggest following the IT law regarding sexually explicit content in connection with virtual worlds.
[d]epictions or descriptions of: (1) sexual references; (2) full or partial nudity, including depictions of uncovered female breasts, aroused or unaroused male or female genitalia, and unrealistic or overly detailed genitalia; (3) bestiality; (4) sexual acts to or with minors (anyone under the age of 18); (5) sexual acts including, but not limited to, penetration/intercourse, and/or oral sex with or without another avatar or any other object, including overt sexual toys and/or sexual aids; or (6) sexual behavior that has a violent context." - Summary List (TL;DR)
There for explicit visual depictions should never be allowed, without proper censorship. Taken that hentai is in fact animated pornography, it is safe to say that hentai is off topic as per site policy, unless censored and answerable in a way to comply with these rules as well, in a scientific way.
Mild / borderline explicit content
(1) sexual references, including (6) scarce clothing, which does not yet include partial nudity. Think of panty shots, skimpy clothing, naked bodys hidden by a cloud. Should be on topic, given that they are contained inside a >! (spoiler) area, and visibly warn the user of their content. This should both count for answers and questions alike. And were possible, especially in answers, avoided if possible.
Graphic content (gore and violence)
As there currently is no policy in place in regards to such graphics contents, and avoiding those in anime is nearly impossible. I suggest allowing this content, if properly hidden from direct few, with a visual disclaimer warning the user of its content, and if possible should be avoided. Especially in answers
Currently there does not seem to be a rule regarding this on the site (correct me if I am wrong). There for I suggest the following:
Uncensored Graphic content such as, depicted murder, assault, suicides and or tortures. Or any other scenario leading to injury, bloody weapons, extensive blood effects or anything similar of nature. Should be contained in a >! (spoiler) area. and visibly warn the user of their contents.
Answers should try to avoid this content as much as possible, but in case this is not possible, adhere to the above mentioned rules as well.
Which I based upon Graphical content as mentioned on wikipedia.