I've been wondering about that for a long time, and it turns out (after cursory googling) that a lot of the hentai mangas that get translated into English are drawn by female mangakas, according to their fans and sometimes the artists' personal pages (which not all of them have).
One thing to remember is that usually only higher quality works make the cut and get translated into English and posted on English-speaking hentai manga sharing sites, where the total ratio of male/female mangakas is not representative of the total number of hentai artists.
As I don't expect there to be enough info out there on the artists who don't make the cut, I'm more interested in the ones that do. But if there are surveys/statistics on the total number, that would be interesting to learn too.
As for what I've tried before asking: I found a couple of big torrents which list files starting with the mangaka's name (or pseudonym), but without ability to read Japanese, I'm unable to research any of them to find out their gender. That, and there being several thousand of unique artist names, is a massive undertaking. So unless there are already some statistics out there, I guess it's gonna take a while to get an answer.