As explained elsewhere, we primates have certain visual cues we rely on to tell a woman from a man; men are generally taller and more muscular. Women are shorter, more curvaceous and have breasts. This is because we are Sexually Dimorphic animals. This is also the case in several other animal species, particularly those who also rely mostly on vision, and communicate largely with visual displays. The most common kind of dimorphism is when males and females of the same species sport different colors and markings. And science fiction writers, when inventing aliens, love to run with this.