The assumption is that biological masculinity and femininity form a spectrum based on the complex interplay of genes, environment, and time, and that, just because you know what genitals someone sports, it doesn't mean you know how their bodies and minds will respond to particular circumstances.

This cuts both ways: you will often see physically emasculated men get treated as if they're not men, to their detriment. Think of less-developed young men being cut from sports teams; they have the same in-built need for physical activity, competition, even some measure of practicing dominance, as larger boys. Deprivation leads to frustration and more.