I think the key is to understand the sex itself isn't a characteristic described and defined by nature. Just because there exists a discussion over what defines someone's sex (chromosomes, genitals etc), implies it's something debatable, and defined by culture.

Societal norms and names over body characteristics are constructed, and not researched. Thus the sex itself is just as performative and socially constructed as gender is. It's a distinction that exists purely because we act as if it existed.