To add, since earlier in the thread you mentioned that sex is biological while gender is cultural, this is an example of why that isn't precisely the case. Sex is also defined culturally, even as it is informed by one's understanding of biological sciences.
Honestly, this is how all human descriptions of anything works. There's the phenomena that exists in of itself with or without the presence of human observers, and then human observers interpreting it and putting labels on it and putting it into boxes. That part is culturally constructed, or has gone through a filter. This is difficult for many folks to understand, but especially for topics they have a personal stake in or are attached to as a map-of-territory.