Aside from grammatical gender, gender as a concept is not even 100 years old. Let's not pretend it is this obvious fact of reality that humanity acknowledges as a given. Most societies throughout history had ideas about men and women (usually if there is an 'other,' it is always only 1 other, and it is always males taking a female role) often with slightly different criteria for what constitutes a 'sex' (biological or otherwise, often moral/religious) and we tend to whitewash that by imagining our modern Western ideas of 'gender' are a reference to those ideas. Actually, 'gender' is inherently suffused with modern Western principles about how to conceptualize sex categories. It is for this reason that 'third-gender' fell out of use in cultural anthropology. By saying that the Hijra in India/Pakistan or the Two-Spirits of various Native tribes had notions of 'gender,' we place modern Western constructs on their practices as if those practices are safely subsumed by 'gender.' In practice, it doesn't work. If you are a Westerner talking to other Westerners, though, and you don't need to actually speak to people from other cultures or have your ideas make any sense to them, then you probably won't run into many problems using 'gender,' but that jars with the notion that any particular amount of genders, or the existence of gender at all, is self-evident.