The only way that trans people make sense is if gender and sex are two separate things.

How is it that I, a straight man, am attracted to trans women if they're actually men? Does that mean that I'm bisexual? No, because I'm not attracted to cis men. (Or trans men, for that matter.) Think about the pretzel logic you have to get into if you have a worldview like JK Rowling. "Bisexual for femininity" isn't a thing. Straight men are attracted to women. Cis women, trans women. Women. Period.

Now, I know that it's about more than attraction, but this is what got me to start thinking about this. Another was this thought experiment somebody gave me: "If your dick got chopped off, would you instantly turn into a woman?" No, of course not.

At this point a gender essentialist would probably start talking about chromosomes. Chromosomes determine what your gender is! Nobody uses this as a definition for gender in any practical way, though. When you're looking at somebody from across the room, you don't know what their chromosomes are. How many of your friends do you have chromosomal data on? How about the people you've dated? Are they going to use chromosomes to determine which bathroom you can use? Or which team you can play on? This is incoherent. It's like basing gender upon whether you have an even or odd number of atoms in your body.

I had one person tell me that a trans woman would be unsuitable for marriage because she couldn't give birth. I'm not interested in having children, but setting that aside...would they say the same thing about a cis woman who's infertile? Every single one of these arguments is incoherent.