Second wave feminism produced the classic 1972 children’s album Free to Be You and Me, which I’d like to point out was not titled Free to Be Me But I Get to Define You.
You don't get to define me. Stop calling me 'cis'.
Back then we thought gender really was kind of binary and defined by genitals; science has gotten smarter in the decades since and we now know it’s a complex interplay of chromosomes, hormones, primary and secondary sexual characteristics and other stuff,
Do you mean sex or gender? You seem a little confused. A dictionary might help.
some of which is in the brain, not the pants,
Never though the person who wrote 'Men explain things to me' would believe in ladybrains. Did a man write that book for you? Obviously your weak and feeble ladybrain couldn't have come up with anything as complex as writing a book.
and also that quite a significant number of people are born intersex, and some are misgendered at birth, and male and female never were airtight categories anyway.
Everyone is either male or female, including intersex people. Why not order a biology textbook along with that dictionary?
Cultures from Native America to India have long recognized that there are other ways to be gendered.
Indeed. Homophobic and patriarchal cultures have often found a way for gay men to exist within their societies whilst being excluded from the category of men, which is reserved for manly, heterosexual men only.