Biological sex is very complex but can appear simple with limited information because (1) many of the features are not superficially evident in gross anatomy, and (2) the majority of the time, most of the features line up in one of two main ways, making it seem like a simple binary thing.
And, in fact, there's research showing connections between transgender identity (where socially constructed gender -- usually based on the more obvious indicia of biological sex -- doesn't line up with gender identity) and instances where the various indicia of biological sex aren't aligned in the usual way. Its not, as I understand, apparently the only factor in transgender identity, but it seems to be a significant correlate.
The idea that gender ought to be simple seems largely supported by the illusion that sex is simple.
And, in fact, there's research showing connections between transgender identity (where socially constructed gender -- usually based on the more obvious indicia of biological sex -- doesn't line up with gender identity) and instances where the various indicia of biological sex aren't aligned in the usual way. Its not, as I understand, apparently the only factor in transgender identity, but it seems to be a significant correlate.
The idea that gender ought to be simple seems largely supported by the illusion that sex is simple.