When I said that in the US gender wasn't used as a synonym for sex prior to the 90s, I didn't mean the word gender was never used. In the US and the rest of the Anglophone world, gender was used in the classic linguistic sense when studying or referring to certain languages like French that designate nouns and related words as masculine, feminine or neuter.

When I said people in the US used sex to mean sex back then, I also didn't mean to suggest no used the words feminine, masculine, femininity and masculinity back in the eras I was speaking of. Those words were in wide use in the US and the rest of the Anglophone world going way back. It's just that no one confused them with sex as often happens today. The way it used to be,

Sex=male/female, a fact of biology determined at conception inside your mother's body.

Sex stereotypes=masculine/feminine, cultural concepts learned and enforced through socialization after birth.

The Feminine Mystique was precisely about how feminine sex stereotypes and oppressive, limiting concepts of femininity had come to be imposed on women, and how unhappy women had become as a result. Friedan was not using feminine as a proxy for sex. The feminine mystique stood in sharp contrast to the female reality.

In the 1960s, sexologists Robert Stoller and John Money began using gender as a shorthand term for the stereotypes of masculinity and femininity that the transsexuals they worked with and studied were preoccupied with.

BTW, I just looked up mystique, and think it's a word that's quite relevant to convos about "gender" today.

Noun: 1) a fascinating aura of mystery, awe, and power surrounding someone or something; 2) an air of secrecy surrounding a particular activity or subject that makes it impressive or baffling to those without specialized knowledge

Today, a very narrow, rigid and (to some) sexy definition of femininity has emerged that could be called the new feminine mystique, and it's this new feminine mystique that trans-identified males seem so entranced and intoxicated by**. Whereas trans-identified females today seem enthralled with a narrow, rigid, cartoonish definition of masculinity that could be called the new masculine mystique. And all gender ideologues seem preoccupied with and utterly under the spell of what could be called the new gender mystique.

** The new feminine mystique of the current era is very different in a number of important ways to the feminine mystique of the post WW2 era that was Friedan's concern.