Reverie, I appreciate you asking this author a question in good faith. I don't have all the answers; I can only speak from the privileged perspective of a cishet woman raising 3 kids: a 20 year old male, a 19 year old female, and a 14 year old trans non-binary person, who was assigned male at birth but was absolutely "hard-wired" from the moment they could walk and talk to be what society considers "feminine." Not only through preference of toys, clothes, characters, costumes, books, movies, friends, etc., but also in behaviors, mannerisms, speech patterns, and every other way you can think of from A to Z.
This child "masked" as a functioning male in society until they couldn't stand it anymore, and socially transitioned over the course of 5th grade in public school, in a very public way that was impossible to *not* notice. They went from wearing "boys" clothes to "girls" clothes, shoes, jewelry, and grew their shaved head out to long, flowing locks (which took years).
That said, their transition was not 100% male to female because they've always identified as non-binary (or, at least, since they learned that word). I don't understand what that gender identity feels like, but I understand who my child is at their inner core and that part has always made perfect sense to me.
I stood back and watched in awe at the sheer bravery of this child, transitioning to live and express more in line with their deeply held sense of gender identity.
People speak of a world where we can "just be" and let everyone else "just be" and all live in harmony, but, unfortunately that's a utopia that doesn't exist... yet. For their entire life and from all arenas - school, neighborhood, church, "friends" & strangers alike, my child was questioned constantly, teased, mocked, harassed, excluded, isolated, bullied, and even physically assaulted for being a "feminine" boy, and later, the same treatment for being a trans non-binary person.
No one except a small handful of us family members have gotten our child's "singular they" pronouns right over the course of nearly 5 years now. People don't understand so they don't bother. If it doesn't affect them personally, in many cases, people simply don't care. I know that sounds like a jaded view of the world, but it has been our true experience going on the better part of 14 years now.
I hope I live to see the day when people are truly free to "just be" their internal identity, without the noise, judgement, microaggressions, and violence of others.
Oh - and to your point of people who are gay or trans not being "scientifically" hardwired in the brain, fortunately, someone cared enough to do research on that. And research exists showing differences between gay and straight, and transgender and cisgender brains. One in particular that I remember from a few years ago shows through MRI scans how a trans woman's brain is nearly identical to a cis woman's brain.