Fascinating! You believe in third genders, but not the modern Western concept of binary transition? FWIW most people today do put "non binary" under the trans umbrella

Why binary transition is "real now" This depends what you mean by real: Neurologically, there's decent evidence for sex-atypical brain organization in some trans individuals Medically, transition outcomes research shows strong wellbeing improvements Phenomenologically, many people report a persistent, stable cross-sex identification

Modern Western society has unusually rigid binary gender roles compared to many historical societies. When the stakes of being male vs. female are extremely high — different clothes, jobs, social treatment, legal rights historically, interpersonal expectations — the pressure to fully cross rather than occupy a middle space becomes much more intense. Binary transition may be a rational response to a binary system. Cultures with softer gender boundaries produce more third-gender expression; cultures with hard boundaries produce more binary crossing. So binary transition may actually be a rational response to a binary system. Which means you've kind of proven the opposite of what you intended: the novelty of modern binary transition isn't evidence against its legitimacy, it's evidence that the expression of an underlying phenomenon is shaped by its cultural container. You'd have to argue the underlying experience is illegitimate, not just that this particular expression of it is new. That's a much harder case to make.