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Gender used to just be synonymous with sex. A sexologist (note that he was a sexologist, not a genderologist or a gender studies professor) believed that sex and gender were different things. He tested his theory on a baby, David Reimer, who had a botched circumcision. Since the penis was mutilated, the testicles were chopped off as well, and an artificial vagina was constructed. Reimer was raised as a female, and Money taught him to be a "bottom", forcing him to take positions with Remier's brother. Money used this case as evidence for his theory that gender identity is primarily learned. Despite this, Remier never identified as female, and in fact went back to identifying as male later in life. He committed suicide after suffering from depression and going through marital troubles.
So just remember, when you use "gender" instead of "sex", you're using the unproven terminology of a mad scientists who performed human experimentation and got away with it.