Emily's Cat
Rarely prone to hissy-fits
I think it'd be interesting to see a hypothetical human society that thought of race as something as fluid or internal as gender or sexuality. I'd read that novel.
I think it's reasonable to say that race is far, far more fluid than sex is. The genes that effect skin tone are, I believe, codominant. Offspring generally end up with skin tones somewhere on an actual spectrum between those of their parents. Most of the characteristics associated with racial phenotypes are "mixable", or at the very minimum, children end up with a blend of those characteristics from their parents.
Sex, on the other hand, is binary. Humans are either male or female. They cannot be both, and there is no in-between sex. Secondary sex characteristics (those triggered by puberty) are dimorphic. Each exhibits a range of size and shape, but there isn't an "in between" for female hip openings, or for male genital length. They are still male or female characteristics. Tertiary characteristics are more bimodal, and can exhibit overlap between the sexes, although they have different distributions by sex.
Gender is the set of social roles and expectations associated with sex. Femininity and Masculinity exist along spectra, but there is also no actual biological foundation for those social categories, unlike sex and race.
Gender as it has been understood up until the recent attempt to redefine it is not internal. Gender is a judgment made by other people as to how well you do or do not fit within the socially defined buckets of gender roles and expectations. It is other people's determination of how feminine or masculine you are. Any person can take steps to try to influence the perception of other people and present themselves as more feminine or more masculine, but at the end of the day, that determination is made by other people.
In this thread using the 'wrong' pronouns is tantamount to slavery.