"... the major expansion in the use of gender followed its adoption by feminists to distinguish the social and cultural aspects of differences between men and women (gender) from
biological differences (sex). Since then, the use of gender has tended to expand to encompass the biological, and a sex/gender distinction is now only fitfully observed" [emphasis mine.]
"... gender has come to be adopted as a simple synonym, perhaps a euphemism, for sex by many writers who are unfamiliar with the term’s recent history."