But the generally the argument is that biological sex isn't mutable but gender is. That if one presents themselves with the outward characteristics we socially ascribe to one gender, they are said gender, socially. Because 99.9% the cues we use to guess someone's gender aren't their chromosomes or genitals but how they present themselves. The way they're dressed, their name, their mannerisms.

Which is why we can look at this drawing, for example, and immediately assume it's a woman even though it has no genitals or chromosomes.

That's why there's a separation between sex and gender and why people believe they can change genders. One is a biological reality and the other is socially constructed concept.