Yes, I note that gender is a social construct, because it changes both over time and across cultures. However, gender identity is a whole other ballgame — and it isn’t “nonsense.”
Gender identity is someone’s deeply ingrained sense of who they are at their core — male, female, some combination of both, neither, or something else altogether. Everyone has a gender identity, and for most of us, the stars align. But children can very much know if who they’re being told they are doesn’t match up with how they feel at their core, and this happens even though children are likely to lack the verbal skills or vocabulary to properly explain who they are (or are not). It’s why so many people don’t transition until their 50s or 60s. I’ve been fortunate to meet many of them. They all have shades of the same story, from every corner of the country, that when they finally learned the word ‘transgender’ and all that it encompasses, everything finally clicked, they suddenly knew who they were.
Gender identity is developed from very early ages, like around 2–4 years old, and is often fixed (at least in outward presentation) typically around age 6 — regardless of whether the child is showing conflict over it, or has learned to hide that conflict.
Regarding sex, there are many different things to consider. For one thing, it’s not binary, just as gender isn’t binary. There’s “genetic sex,” meaning chromosomal inheretance such as XX, XY, XXY, XYY, XXX, XXXY, etc. (On that note, X & Y chromosomal variations are common, but frequently undiagnosed. Many people don’t find out until trying to conceive and can’t, ultimately learning they’re infertile due to being intersex. Others may go to the grave without knowing.)
Then there’s “physical sex,” meaning one’s primary and secondary sex characteristics — penis and testicles; vagina, ovaries, and uterus; or, as in intersex people, any combination of these. Like having a penis and ovaries. Or having a vagina, but the chromosomal makeup of a male.
Most people have difficulty accepting that sex is not binary — because that’s not how they learned it — but regarding gender, the U.S. is woefully behind on grasping gender beyond the binary. There are many cultures around the globe who have long-recognized, and even celebrated three or more genders. Like: Native American “Two-Spirits,” Hawaiian indigenous society (Kanaka Maoli) “mahu,” “Bakla” of the Philippines, or Indonesia’s “ Calabai, Calalai, and Bissu,” to name just a few.