Volodja wrote:
I'm assuming it means gender rather than sex. Although if it means sex then that's even more reason for it to be optional - someone's sex is nobosy else's business
You are uncritically equating "sex" with sexual behaviour. I am referred to physiological sex.
Gender is a grammatical term that refers to the classification of nouns into 'masculine', 'feminine' and 'neuter.' It in only with the squeamishness of Americans towards the word "sex" in the last 50 years that the gender has come to be used as a term for the biological sex of an organism. (The first unequivocal OED reference to "gender" used to refer to biological sex dates back only to 1963).
From a medical perspective, there has been some use of "gender" in place of "sex" when referring to psychological identity. So, for example, "gender dysphoria" is an accepted diagnostic term where the physchological and physiological sex of an individual are not aligned.
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--James