Are you suggesting that "transracialism" is treated as science or supported by anything approaching the mainstream left?
I think this is part of why some conservatives find themselves unwelcome. Being outwardly disparaging toward people does not make you friends.
It may be harmless (as you correctly point out), but it is not tethered to reality.
Rachel Dolezal (by far the most famous case that I know of) was pilloried by the left for cultural appropriation:
https://www.google.com/search?ei=x-UhWuymEIyojwOc6Kz4Bw&q=ra...
Shouldn't there be universal condemnation?
"The left" is not a spec or rigidly defined ideological canon that all "leftists" conform to. As with "the right", "capitalists", "socialists" or any other political or cultural generalization, actual "leftists" can comprise a spectrum of acceptance or disagreement.
Remember, Trump was a Democrat for a long while before he was a Republican. (IMHO, he became a Republican solely because it was the easier party to hijack at the time.)
For instance, all the sex-and-ethnicity choosing going on. Totally without scientific merit, yet the left behaves as if it's science.
The pendulum swings both ways. Trump's election is a direct result of left-side hysteria and over-reach.