Because it is considered cool right now.
There are several factors involved.
Dr. Deborah Soh said there can be an element of homophobia involved. Your daughter who might have been a butch lesbian is now told she’s a boy. An effeminate boy who would have been the feminine half of a homosexual pair is now told he’s a girl. I don’t think this is the case but can understand her point of view.
I think it is more a matter of incredible sexism on the part of the left. By saying everything is IDENTITY, your behavior matters much more.
The glory of the American identity is that it is ideological - every race, every creed can join. The downside is that there is a strict checklist of values you have to share or else it all falls apart. The same is true of religion - if there is no tribal demographic aspect, cohesion requires strict compliance with rules.
Liberals are struggling with ethics based on elevating up the oppressed. Whoever is the most exotic is therefore the most correct in their demands. There are fewer transgenders than homosexuals, fewer homosexuals than heterosexuals, so the most extreme sexual behavior has to be catered to.
Homosexuality required saying that gender doesn’t matter in marriage - all you need is two people who love each other. We quickly got calls to endorse polygamy and homosexual “throuples”.
Transgenderism takes it further. How do you cater to someone who says they’re the wrong sex? And 80% of the men choose NOT to have the plumbing removed? The solution is to have a strict “identity” checklist. If you act feminine and sound feminine and spout feminist dogma, you’re a woman. If you don’t act feminine but are a woman, we’ll say you’re really a man. If you’re a feminine female but not spouting female dogma, you’re not really a woman.
The practical impact is the demand to accept a man in a dress in women’s bathrooms, locker rooms and dorms. It is telling lesbians you’re a bad bigot if you won’t have sex with a man who identifies as a woman and is still attracted to women. (A mentally ill heterosexual.)
Liberals’ identity politics ties ideology to tribe. If you want to be a member of X tribe, you have to live out the stereotype. Real blacks, Asians, etc eat certain foods, wear specific clothing, hold beliefs liberals ascribe to them.
Transgenderism is the application of that same merger of political dogma with tribe. That if you meet all the criteria on the checklist, you’re a member of the tribe. If it were a Native American tribe choosing to adopt someone, it is like a religious conversion or citizenship - they make that decision and you can join.
Biological sex is irrelevant when “identity” is paramount. In fact, they see it as a moral good. They’re denying biological reality in defining who is a woman and who is a man. And they think that doing so MAKES them good.
Liberal politics has now reached the point of a religion, political dogma invested with religious levels of devotion. If the man says he’s a woman and fills all the appearance and behavior checklists, it’s a miracle, he’s a woman and must be celebrated as stunning and brave. She identifies as a man, hallelujah, we’ve proven that there’s no real difference between men and women and feminists are proven right!