In that metaphor, why is gender part of the type hierarchy in this context? Why is that the first-class classification for concrete implementations of `Person`

In that metaphor, why is gender part of the type hierarchy in this context? Why is that the first-class classification for concrete implementations of `Person`? Why not hair color? Why not height? Why not age? I feel like everyone would have been better off if pronouns weren't even gendered to start with. But now that would be an incendiary political crusade, which I'm not interested in.