My take on this: that question was closed incorrectly, twice. Our community has a fairly clear consensus on what is and isn't allowed. We've amassed the 5 reopen votes needed to reopen the question, and that too was ignored. (For kicks: the total reputation of those voting to reopen here is 41,499, and while that obviously shouldn't factor into official policy at all, it's an indication that the people reopening here likely know what they're doing.) The moderators here are acting rogue based on their own objections to such material rather than on any site consensus or actual SE policy.
As for the general case, questions about explicit content are allowed here, at least based on the previous consensuses. But questions which are themselves explicit are not. A well-made post can avoid violating the ToS, the Content Policy, and the Be Nice policy. So long as you keep any discussion of sexual content to a minimum, use clinical language, and avoid obscene images completely, such a question is acceptable here. In general, this isn't usually very difficult unless your question would already be inappropriate here for other reasons (e.g. a recommendation question).
I can find nothing in the question there which is remotely problematic; the only words with sexual content are "love hotel", "suck", "impotent", "seme", and "uke", and none of these are very problematic. Sure, graphic descriptions of sexual acts would be a problem, but there is none of that here. This is almost a perfect example of how to write a question about a hentai; the only problem is that as an identification request it only barely meets the requirements. The moderator's closing of the question because of his own objections to what he believes the answer will necessarily be come off as absurd and juvenile.
In fact, the objection here demonstrates a serious misunderstanding of hentai itself. Equating "hentai" to the western concept of "porn" is completely misunderstanding many things. "Hentai" includes all series that explicitly show sexual activities. Most of these have a lot more story and characterization than western pornography. It is better to identify hentai with the western concept of "adult entertainment" which while literally synonymous to "pornography", has a completely different connotation. Many hentai like Kite or Bible Black have serious stories and very clearly can't be simply equated to "porn" without missing the entire connotation that carries. Nothing in the question here would indicate to me that this manga is any different; in fact, most of that post describes the story rather than anything sexual.
And it's worth mentioning that adult entertainment of this type is on topic on Movies SE, under exactly the same rules as here. In fact, they have a tag for such questions, which isn't exactly complete, but at least demonstrates that the community there isn't actively removing such questions just because of their subject matter (as the new "policy" would seem to be here) but only because of the actual content of the question.
Since I know people will complain about this point unless I mention it, let me just say that the post at Meta WebApps is not SE global policy. It was posted as a proposal by a single (former) employee on a single child meta site, and doesn't hold any more weight here than it does on Yahoo Answers. While it's clear that the WebApps community is okay with it, our policy can differ (and already does, in my opinion). SE global policies are those which every site is required to follow, and those appear or are directly linked to on this site itself, e.g. the Be Nice policy in the help center or the legal link below. Anything else, on Meta SE, or other child meta sites, or (obviously) private discussions with anyone, while potentially useful as a guide, is not SE global policy, at least in terms of moderation.
Furthermore, we've had employees on this meta site before discuss this very topic, and they did not state any such global policies. The most prominent is this one by Tim Post, which says that we should seriously consider whether we want to allow hentai, but not that it needs to be banned to be compliant with SE global policy. His response is more along the lines of "children as young as 13 use these sites, and please keep that in mind". However, nothing in the closed question would be inappropriate for children as young as 13; by that age (at least in much of the world including the U.S.) such children would have learned quite a bit more explicit things in school, and the content of the question wouldn't pose a problem. The manga itself might not be appropriate for 13-year-old children, but then again, neither is something like shingeki-no-kyojin, which has gratuitous gore and violence. So long as the question and answer themselves are not problematic, we don't need to be arbitrating the content of the original works.
Indeed, quite a few other sites in the network do allow discussion of adult/sexual matters. There was a Sexuality beta site which failed for unrelated reasons, and there's a proposal for a similar site now. Both of these were approved by the community team, meaning that there's no network-wide ban on discussions that happen to be somewhat sexual. Here's what the current CEO of SE says with regard to a proposed Sexuality site:
[The] site in question will be "safe for work" and require the use of clinical language, which is already the policy when discussing questions of an adult nature on other sites in our network.
That is, as best I can tell, the most official response for how adult questions are to be handled, and he didn't seem exceptionally bothered by the presence of such discussions when done appropriately. (Of course, it would be alright to ban such topics with community consensus, but as our community has clearly stated since private beta and repeatedly confirmed, we do want to allow such topics here.) So I don't agree that SE policy requires us to ban these questions.
In fact, we're just finishing an official bounty event: May 2015 bounties for tags: visual novels and adaptations thereof. Of the 59 series listed there, I counted 36 (over half) which were originally 18+ eroge, and several have never had any all-ages releases or non-hentai anime adaptations. A couple, like Wanko to Kurasou, are basically nukige, which is about as close to the western concept of "porn" as visual novels get. It's hypocritical to be promoting such content on the one hand, while simultaneously blocking it on the other. Given that this event was created by another moderator, it seems like there isn't even a clear consensus among the moderators themselves as to what is allowed, which is frankly a moderation disaster if policy is being implemented on the site itself single-handedly without even discussing it with the other moderators first.
Perhaps more importantly, it's essentially impossible to have any serious discussion or Q&A about anime and manga without at some point mentioning hentai. It's rather important to actually understanding the industry as a whole, even if you never watch one. About 10% of all anime on MAL are rated as hentai, and that number is probably somewhat low. Manga are harder to classify, but adult-oriented manga are quite significant as well. Of course, for visual novels, the fraction is even larger, well over 50%. And while the raw numbers aren't so huge on its own, quite a few hentai works have had impact even among more mainstream works. I'm not aware of any "professional" sites for anime or manga which disallow hentai; various sites like MAL, ANN, AniDB, Hummingbird, etc. all allow it (within reason) because it's a crucial subset of anime and manga. The impact is to the point that I'd seriously question whether anyone can be considered an expert on anime or manga without ever having looked at such a work.
If anything, I'd say that hentai are something we ought to try harder to cover more completely. Right now, we're doing a terrible job at covering these series, and the new "policy" isn't going to help anything. As I understand it, the goal of this site is to become the best place to ask good anime and manga questions about all types of series, and we can't hope to be that if we're blocking a bunch at the start for silly puritan, holier-than-thou objections.
The other, most serious practical objection I've heard to allowing hentai is that it will get us blacklisted. Realistically, as I understand it, there's little to no chance of Google blacklisting us. Google is really only concerned with blacklisting sites that are suspected of distributing Malware; plenty of strait-up pornography sites are still indexed. I only know of one case where a site was blacklisted for having sexual content, and that was only for 1 day before it was removed. The site in question specifically focuses on such things, and is still indexed today; I won't link it here but it's a very well-known blog in the anime-sphere.
Realistically, we're never going to get that bad. We probably won't get close to the 10% of all content being hentai-related that MAL has. I suspect that the level here will be 2-3% of all questions at the most, at which point this isn't even a remotely realistic possibility.
There are various internet-filtering software which might identify this site and blacklist it, but those are used by a minority of all users, and it's still quite possible for this to happen even if we block hentai completely. Most of this software is configured so that people will only go on work-related sites at work. As this is an entertainment-oriented site, there's little reason to be on it at work anyway for a majority of viewers. Hence, I don't think there would be major consequences for being blacklisted in such software. In any case, while I don't understand the exact methodology used by such software, the use of clinical terminology would presumably go a long way to avoiding being blacklisted, and beyond that I don't see a good reason we should cater to a small minority who use this software rather than a sizable portion who watch and enjoy hentai.
This is a site about anime and manga, and that obviously includes hentai. Whether or not you like it individually, we as a community need to own that, not bury it. I'm happy to do my part in this, as are quite a few other users; I'd ask that moderators not screw it up by implementing new policies out of nowhere without first seeking community consensus.