Yeah, sure, and for most people it’s a fair enough proxy. But if it has to be boiled down to exactly one of “M” or “F”, then “birth sex” must not be the deciding factor. If it must be a single criteria, it should be current hormone levels, artificial or not. And, since most trans people who actually transition and live as their preferred gender identity are on hormones, “gender identity” is a good proxy for 99.99% of the population, including the set of people for who “birth genitalia” is also a good proxy. But ideally, it doesn’t get simplified this much in the first place. And of course, it doesn’t, in practice, because most people actually form a relationship with their doctor, and they treat holistically, based on individual factors, and not simply whether the medical record says M or F.

But, if we must over generalize, “gender identity” really is the most useful proxy, in fact, and it also happily happens to be quite inclusive too.

Of course this conversation started from a transphobic viewpoint, which doesn’t actually care about any of these distinctions anyways, regardless of the merit, it’s just someone being triggered about someone respecting someone else’s gender identity.