Gender identity language was invented because people disliked saying sex
Andrew Gold said:
“My understanding was we started using the word gender because people felt a bit funny about saying the word sex, they don't like saying this, they use gender, so it's just a synonym for sex.”
It sounds like a plausible piece of social history: euphemism is a real phenomenon in language, and many people do use sex and gender interchangeably in everyday speech.
The distinction between sex and gender was not coined because people were squeamish about a three-letter word. Researchers in medicine and psychology developed it deliberately in the 1950s and 1960s precisely to separate biological characteristics from the social roles and psychological identity that do not map neatly onto them, and feminist theorists took it up for the same reason. The distinction did real intellectual work, which is why it was eventually written into international health classifications and into UK law, where the Equality Act 2010 lists sex and gender reassignment as two separate protected characteristics rather than treating them as the same thing. When the two are folded back into a synonym, every policy and clinical conversation that depends on telling them apart becomes impossible to follow, and the people navigating those conversations in real life are the ones who lose.
Anyone trying to understand the policy and clinical debate, because collapsing the distinction makes it impossible to reason clearly about what is actually being discussed.