It is completely astonishing that people are trying to argue that sex is not binary.*
This is not my area Pratchet, but it seems quite clear to me that for the purposes of reproduction a binary is required - and exists.
But sex in the context of biology is more than just the means, behaviours and equipment required for reproduction.
So I think what's being studied is that the characteristics or sex - physical, behavioural or whatever - may present on a spectrum, the broad two halves of which represent the biologically dimorphic nature of our species (as required for reproduction) with a bit of a hazy grey area in the middle.
Like I say, not my area, just what I understand about the question being raised.
Everyone knows that for the purpose of reproduction there are two required body types - fully functioning - one act to be committed, and a selection of behaviours to be exhibited by each party to get them to commit the act in the first place. I don't think a living soul disputes that.