Man of Honour
How is that a "fact" why are gender and sex not the same thing?
By definition. Sex is physiological. Gender is anything in any way associated with sex. Two different ideas, two different words. It makes more sense that way.
For example:
Pink in the UK used to be gendered strongly masculine then was changed to being gendered extremely feminine and is now gendered very feminine with some attempts being made to gender it less feminine.
Pink doesn't have a sex.
The problem is that people often mash the two together, saying things like "pink is a girl's colour". Even many people who say they're not the same thing still often talk about them as being the same thing and that's becoming much more true due to the current politicalisation of sex and gender. So, for example, most people refered to as transgender are actually transsexual and that's a very different thing. This pressure to confuse the two stems from and ties in with feminism and other fashionable irrational prejudices, by associating as much as possible with the biological group identities that believers in such ideologies believe in so devoutly and by shoring up the belief in simple discrete groups (as required for belief in biological group identities).