“This is really the natural devolution and crumbling of gender boundaries that started in the fifties and sixties when women started wearing pants.”
I don’t know how old you are but I suggest you remove the 50s from your statement!
I remember it happening in the sixties, but it started (at least in my area) a little earlier than that. When my parents were growing up a girl would not dream of wearing pants or slacks out in public. Jeans might be worn if she were working in the field in slopping hogs.
I found a letter my mother wrote to my grandmother in the early sixties, when I was a baby. She referred to a woman she just met who wore pants and smoked and it was considered so gross and unusual.
I also recall in the seventies in school girls could not wear pants unless it was under freezing—and the weather was checked every morning and girls who tried to get away with dressing like boys were sent home. And female teachers were never allowed to wear pants or slacks.
That’s where it all started—that’s what brought us to where we are today, boys wearing dresses to the prom.