Gender is SO hard to untangle because it means so many different things. Most people have an innate sense of what gender feels right to them. But then that gets complicated. We have to deal with sexism, what we wear, how we act, what we look like, what kind of bodies we have, gender roles, expectations, medical care, and legislation.
I spoke to an older woman the other day who couldn’t have her own bank account or credit card while she was married to her controlling ex-husband. It wasn’t until 1974 in the United States that the Equal Credit Opportunity Act granted women the right to their own credit cards separate from their husbands. Patriarchal ideas of gender have been used to control people for a long time.
(The Equal Credit Opportunity Act protects you from being discriminated against by lenders based on race, color, religion, national origin, sex, gender, marital status, age, or being on disability or SNAP benefits.)

A particular combination of gender and age—women, in their 20’s and 30’s—are seen as the archetypal woman, perhaps interested in dating, marriage, forming a nuclear family, and raising kids. For people who are not interested in this domestic career trajectory, this expectation from society can be obnoxious.
Quite a few people try to talk me out of being nonbinary in the comments on my videos on YouTube. (It hasn’t worked yet.) Because I “look like” a woman in my twenties (I’m 33, but I look young), I represent a threat to men who assume I should be someone they or other men can control. Instead I am demonstrating an unconventional life, with the freedom to go wherever I want, say whatever I want, and date whoever I want.

What if a lot more people were inspired to live the life they want? Who loses power, control, and free labor? Does the patriarchy fall from power?
We suffer through all these subtle reminders that we should be the right gender and age to optimally contribute to society. That would be women age 18-40 and men age 18-75. Then our society just pretends any other type of adult doesn’t exist.
This is related to people calling old women “young lady,” which I wrote about here. Correct me if I’m wrong, but I don’t think people generally say “young gentleman” to old men. There isn’t an age cutoff for “being a gentleman” in the same way as “being a lady.”