By
rigid I mean to imply also a sort of moral commitment to keep people in their assigned lane.
What the spectrumists mean by "viewing sex as a spectrum" is confusing and often unclear, but I think
Novella gets it right when he says "the development of sexual characteristics...can vary along a spectrum," which is to say that individual sexed
characteristics (e.g. breast development, laryngeal development) may vary continuously from clearly male to clearly female in size/shape/appearance. Rigid binary thinking (of the sort demonstrated by the Catholic bishops) is that someone biologically destined to have an ordinary male chest and Adam's apple mustn't be hormonally or surgically assisted in their desire to change those specific outcomes to something which we'd expect to see on a female.