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.
Who Owns A Recipe? Race, Food And The Debate Over Cultural Appropriation
I think the metaphor is useful: if you don't change your culture regularly, it starts to stink.