Even when it comes to sex, there are more biological sexes than male and female! But in terms of gender, there are men, women, and a pretty big spectrum of folks who identify somewhere outside of that framework. Everyone who isn't a man or a woman will have a pretty personal and unique understanding of what that means; it isn't that  there's a secret third gender but more that there's a pretty big universe of genders and that "man" and "woman" are the ones people have the clearest framework for right now, if that makes sense.