How is it that gender is considered fluid, but culture is fixed to a person's skin color?

HINT: Culture is highly fluid

It is the net accumulation of continuously re-negotiated social contracts between millions of individuals.

All I can think of is a documentary on Dark Age "Realy Boring Age" Britian in which they were showing a bottle of Garum with "Product of Tunisia" on the side. They were discussing how Garum lead to the most English of Sauces: Lea & Perrins.