Unfortunately, that's a limitation of the forum engine we're using. I did a check for what mods might be available that would allow additional gender options, but I didn't really see anything for it. We can keep this feedback in mind, and perhaps an appropriate mod for this will show up someday. But for now, we're stuck with what we've got. My best advice, if you want to be sure people know how you identify, would be to put something about it in your signature or personal text for now.