“Intersex people prove that sex is a spectrum, not a binary.”
There are good scientific and philosophical reasons to reject this idea. To quote at length from Colin Wright and Emma Hinton in the Wall Street Journal:
In humans, as in most animals or plants, an organism’s biological sex corresponds to one of two distinct types of reproductive anatomy that develop for the production of small or large sex cells—sperm and eggs, respectively—and associated biological functions in sexual reproduction. In humans, reproductive anatomy is unambiguously male or female at birth more than 99.98% of the time. … There is a difference, however, between the statements that there are only two sexes (true) and that everyone can be neatly categorized as either male or female (false). The existence of only two sexes does not mean sex is never ambiguous. But intersex individuals are extremely rare, and they are neither a third sex nor proof that sex is a “spectrum” or a “social construct.”
Regardless, debates over the precise nature of biological sex are ultimately irrelevant to the issue of transgender identity. The vast majority of transgender individuals are not intersex, and it’s unclear why the existence of intersex people would validate unambiguously male people claiming to be female or vice versa, any more than the existence of biracial people would justify an obviously white person claiming to be Black. After all, there are countless social and biological categories that undoubtedly exist along a spectrum: Young or old, tall or short, rich or poor, conservative or liberal, and so on. But no one in their right mind would argue that the existence of the middle-aged or the middle-class implies that the elderly can identify as teenagers or that billionaires can identify as impoverished!
This basic idea – that two groups can’t be considered distinct just because some gray area exists between them – is sometimes referred to as the continuum fallacy, and philosophers have recognized it to be an example of bad reasoning since the time of the ancient Greeks. So while discussions of human development may be valuable for their own sake, the implication that they have any bearing on the validity of transgender identity generally is just illogical.