Oh, well that particular double standard is because the left has created this weird mythology behind every race. White people are the evil empire. Black people are all rapping, basketball playing teenagers constantly running from the racist all-white police force. Mexicans are hard working farmers who just want to go home and eat tacos while listening to the Mexican Hat Dance.

Any time anyone attempts to eschew their assigned cultural role, leftists get extremely uncomfortable. This is because they have this idea that your race determines your life trajectory, from how you're born to what you'll like to how you'll die. It's fatalism, and free will really doesn't mesh with their belief system.

Gender is viewed the opposite way: an absolutely meaningless descriptor that determines nothing whatsoever. This is a side effect of years of being told that girls can do anything boys can do, which stupid people interpreted to mean "boys and girls are literally the same". As such, you can freely swap back and forth without causing them any discomfort.