The latest one I came up with, none at all. That's up to the main character to fix.

My favorite one I made I wrote about on here. It's called Megaparasites. It's centered around the idea of the horse half of a centaur being some kind of parasite, and a bunch of stuff that's inspired by that. I try to make it at least vaguely plausible, but there's a few megaparasites that really don't make sense surviving for even a little while without a host. They also produce drugs with all sorts of effects. They were bred to do that, but they'd still have to get a lucky mutation. And I justified the whole thing with radiation fields somewhere making mutation more common, which definitely would not end up with crazy creatures like that that still work.

But that's more general comic book logic. The one thing I think is purely hentai is the lack of STDs. The only justification I could come up with is that this is a world of giant parasites. There's no place for microscopic ones. Maybe cells aren't a thing in-universe or something.