The problem here is hentai content based of anime characters. Most of these characters are under 18 canonically and thus any erotic content wouldn't be allowed under current ruling even though they look like and act like adults.
Then there's the gray zone for characters that have progressed or regressed in age in some way, a good example is Ciel from the visual novel Tsukihime who technically is past her 20s, but her body stopped aging at 26 due to lore reasons. Under current ruling it would be hard to be certain whether it's allowed or not even though she both looks and act like an adult (although a weird one as Nasu can't write a proper R18 scene to save his damn life). Another issue is aged up variants of characters where their canonical age is way below 18 but the art depicts them as above it.
That said I do stand by enforcing an age limit for real people, but when it comes to fictional characters (and anime characters in particular) it makes more sense to go by appearance, this rule would include all these characters while still disallowing Loli/Shota which is the main concern for other instances (see burggit for example). This draws a clear line with less ambiguity than "they are a child" as not everyone may have the same opinion and this could lead to more things getting hit in the crossfire than is necessary, yes I know this leaves characters like Megumin in the gray, but it's not as big of a gray zone as "not a child"
Redgifs doesn't embed on this site, either nobody figured out how to get the raw media file, my recommendation would be to use catbox.moe or some other direct media host that doesn't mind hosting for other platforms
For my communities (!gts@lemmynsfw.com and !tsf@lemmynsfw.com) I'm not going to say a strict "no" just yet and take a bit of time to monitor; but For me personally I would rather keep AI in it's own corner (the dedicated AI comms). once either of these get some userbase and more regular posts then I do want to see whether we should allow it (under proper tagging), move it to a dedicated comm or just put a full stop on it altogether.
My big concern is that AI generated content does not respect the authors of the original works it used to train and most checkpoints use various aggregator sites like Danbooru which already exist in a weird state legally. Another concern is that AI art is stupid easy to make (any bloke with a recent NVIDIA GPU can spin up Stable Diffusion or you can use one of many online services/spaces) and start spamming your comm with low effort content. Granted some AI outputs can look good; but most of the seriously good outputs have been selected and altered in some capacity to address the current limitations of the technology.
That said I am not active in this comm and I'm talking moderation standpoint first, lemmy user second, what decision is made should be left to the people who are part of this comm and I have no reason to meddle in it.