We've debated genre tags like this before in one of the chat casts. In that case I think we were discussing lolicon, but the same logic applies here. The consensus at that time was that a genre-based tag could work, but it would need to be used on questions about the genre itself (e.g. production, tropes, etc.) and not questions about individual series. So in this case, What was the first Hentai ever made? could be tagged hentai, but
What is the song played in Operation 11: Fallen Angel Rina? should not be. We didn't discuss identification questions (e.g. Anime identification request about guy with invisible pills (hentai)), but it seems best to also leave these untagged.
The reason for this is fairly simple. Use of the tag in the former way is perfectly fine, since it's about the content of the question. However, the latter style of tagging describes the genre of the series, which is almost always irrelevant to the actual content of the question. Tagging series questions with genre tags would make the series tags redundant and the genre tags bloated, facing essentially all the problems of meta tags. It's also not clear exactly where to stop. Should every question tagged fate-stay-night also be tagged hentai? Of course not, but the original visual novel certainly qualifies as an H-game.
So, if we do make a hentai tag, I'd say it should be for meta-questions about hentai as a genre, not for any-and-every question about hentai series. That may not be what's being proposed here, but it's the only way I can see this tag being a useful non-meta tag. Actually policing tag use to make sure it happens that way is a nontrivial amount of work. And at this point, I simply don't see a lot of questions like this (there are a few others, but less than a dozen), so I don't think the tag is worth it right now given the amount of extra work it will cause. It could become worthwhile in the future though.