There are a couple of elephants in the room which they keep refusing to properly address; they do sometimes acknowledge them but dismiss them as unimportant to the conversation at hand. Which might sometimes be fair, but at the same time, it brings up meta-level issues about the type of society which they demand that no one seems to be discussing.
One is specifically mentioned above:
There are a few cases where an individual does fall into a grey zone, e.g. Turner Syndrome […] who could technically be said to be of neither sex. But thats absolutely irrelevant to transgenderism.
You can’t have a society which enforces an ultra-strict sex binary without *some* answer how to deal with intersex people who still fall into a gray area by whatever standards you draw the line. “We’ll figure it out later” is a tacit admission that the foundations behind your agenda are incomplete. Continually brushing it aside as unimportant is tacit admission that your proposed binary isn’t quite as strict as you’re trying to portray it as.
The other elephant is that the vast majority of human interactions are social, not sexual or reproductive. Humans don’t socially interact with each other on the level of genitals or genes or gametes or whatever standard which they come up with next, even if those are presumed - where it matters, they interact according to gender, or at least gender presentation and/or gender roles (and for some people today, gender identity as well).
In order to enforce an ultra-strict sex binary, you’d also have to either enforce an ultra-strict gender binary (which really puts a damper on the common “compromise” that trans people can just be gender-nonconforming) or you’d have to permanently brand, tattoo, or otherwise mark people in a highly visible place so you’d know at a glance how to treat them and what pronouns to use. Or I guess that one could do some sort of less-strict enforcement where it doesn’t matter for one-time or occasional interactions with strangers, but otherwise one must make clear what their assigned birth sex is to everyone - something which really wouldn’t go well with the “I shouldn’t have to state my pronouns/identity, you should just *know*” types.
…Ultimately, none of this really matters in the vast majority of cases; their only intention is that trans and non-binary people disappear from public life, and they don’t really care how. They’re not invested enough in any one idea to develop them fully.