Here’s my 2 cents. I’m an American millennial, for reference.

Our species has two sexes that are physically different from each other. Neither one of these sexes can transform into the other.

Gender is, first and foremost, a linguistic term. Due to chicanery over the past few decades, it had also come to mean the stereotypes/expectations societies place upon an individual based on his or her sex. For example, the idea that boys like blue, play football, and wear pants OR that girls like pink, dance, and wear dresses.

I think the latter concept of gender is mostly bullshit. Liking baking and skirts is not what makes me a woman. Being female is what accomplishes that. Likewise, being into math and weightlifting doesn’t make me a man or less of a woman. (And if a dude wants to wear a dress and full face of makeup it doesn’t make him less of a man.)

That being said, I think there are probably some traits of each sex that do have a biological basis. For example, aggression levels and risk-taking behaviors tend to be higher in male primates than they do in female primates. Things like this deserve fair and unbiased study to see if there is a biological component. If there is ever concrete proof of explicit differences, it would not mean that individuals falling outside the norm aren’t their sex. It would means they have a feature atypical for their sex. (This is as silly as saying a female 2.1 meters tall has a male height. She does not. She has a height uncommon for females and more typical with males. But she is still biologically female.)

Sex is not a feeling. It is an objective, physical reality that we live with every day of our lives from birth to death. Our sex impacts how we navigate the world. It often impacts how we’re treated, what’s expected of us, and can impose limits on us. Claiming that stereotypes define a person ignores the biologic and cultural realities that come with sex. It is an insult to any thinking person.