The Illusion of Sex demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.
I think the point is merely to identify the subtle visual cues that we use to categorize people into male and female categories. The fact that the more "feminine" face has more prominent lips and eyebrows against lighter skin corresponds to the makeup that women wear to look more feminine: lipstick, eyeliner, eyebrow pencil, foundation for smooth skin; by comparison then, masculinity would be visually defined by less prominent facial contrast. This illusion doesn't define sex so much as it identifies how sex is
perceived, which would be derived from one's culture or environment rather than their biological makeup.
In that video I posted earlier, this illusion would be akin to "gender role," or gender expression, which is defined externally.