As far as I know, this site does not have a policy against posting of any sexually explicit content beyond the Stack Exchange content policy, which states that:
Sexually Explicit Material. Accounts that use Stack Exchange to post sexually explicit or pornographic material, or links to it, will be suspended.
This is rather vague to me. Certainly, pornographic images or links are included, but it's not clear to me that written descriptions of sexually explicit events qualify in all cases. For example, I'd expect a textbook explanation of sexual intercourse would be allowed and on-topic on sites like biology. Given that SE was willing to create a sexuality site, it's pretty clear that the policy isn't just "any mention of sex is against the rules". Exactly where the line is drawn is up to SE, but I'd assume that we want to be as lenient as possible on this while remaining compliant with the SE global requirements.
On the other hand, we've already discussed whether asking about Hentai is allowed, and the strong consensus was "yes, so long as it is done in a 'scientific' way". While that post is about whether the topic is allowed at all, the answers also suggest a particular policy for policing such content. Specifically, it could be inferred from the answers there that, so long as the question is written with "grown-up" mature language, it should be allowed. We've had a few good questions about hentai series as well.
So, in my opinion, unless and until we hear back from Stack Exchange directly for clarification on exactly what is covered by the content policy, we should proceed with allowing questions about sexually explicit material so long as the questions themselves are not sexually explicit. Questions which do not have pornographic images/audio/links and which use the most scientific language available when mentioning about sexual events should not be a problem in my view. For what it's worth, that seems to be the policy on Movies & TV as well as on the failed Sexuality site. If a question fails to meet these (and can't be fixed by slight changes in language) it probably qualifies as spam/offensive and I'd think that in most cases having it deleted that way would be preferable to having moderators enforce the policy.
In the interest of users here, it would be good if such posts generally have some sort of indication or warning so that users who do not want to see that content won't accidentally do so. I'm not proposing this as an official policy, but as a strong suggestion to askers. The warning could be in the title or at the top of the question, and could be as simple as e.g. including "hentai" in the title. Use of spoiler markup as suggested in Braiam's answer is also probably a good idea.
For the question which lead to this meta post, in its current form I would say it is allowed, and actually a very good example of exactly how one should ask an ID-request question about a hentai series. The images (in their current form) are censored appropriately. The text uses the terms "fellatio" and "masturbate" rather than more vulgar equivalent slang terms. The title of the question has the word "hentai" which serves as a warning to anyone casually browsing the site. In its current state this should be unlocked. The history should also be wiped since that does contain images with nudity.