Yes, ChestnutStuffing that was the issue I had to overcome before I got through to DS1. He believed that this was about a person with a DSD who would - via genital surgery - turn into a proper sex. And he could not wrap his head around how I could possibly be so mean as to deny them their sex.
So I had to first explain the difference between people who identify as trans and people who have a DSD.
He's in the age group where this thinking that we are cruel old-fashioned bastards who want to take people's rights away has taken hold, but he just couldn't quite make that fit his knowledge of me (I'm far more left-wing than my kids). So he was willing to listen and ask questions.
DS2 is more difficult, because he's been taught the doctrine of gender identity at school. He claims that he's also been taught complete bollocks in biology about sex being a spectrum but I cannot quite believe it. I'm more inclined to think he picked up that notion in his online spaces and from his friendship group (his ex-GF, the only girl with short hair at school, has been identifying as trans for about two years). We've also got a lesbian girl in the family who identifies as trans. He's been taught that I am mean just asking questions about this. He keeps bringing me gotchas off the internet, but as the arguments are so poor, he's not managed to get me yet. It's almost funny how disappointed he gets, because these online spaces present the gotchas as unanswerable when they are anything but.
I think my youngest has escaped the indoctrination by dint of hearing all about this for the last four years, so when it started he didn't accept any of it. He may well change his mind later, as he has, of course, classmates who identify as trans (no lesbians anymore though), but he's very STEM-focused and logical, so I remain hopeful.