... and what on earth do you do when an obvious male is wearing a dress? ....
Clearly a woman ...
That's the problem - see MW's usage note:
In this dichotomy, the terms male and female relate only to biological forms (sex), while the terms masculine/masculinity, feminine/femininity, woman/girl, and man/boy relate only to psychological and sociocultural traits (gender).
https://www.merriam-webster.com/dictionary/gender#usage-1
"male" and "female", and "man" and "woman" are being used for both the "biological forms" and for the "psychological traits" completely divorced from the reproductive traits. Hence the justification for drawing a line in the sand with the stipulative definitions of Parker & Lehtonen.
Do you think cops always do a complete fertility examination before describing people in their notebooks?
Do you think they check if those people "have developed anatomies for producing either small or large gametes, regardless of their past, present or future functionality" - as per Hilton's tweet - before referring to them as "males" and "females" - respectively? Do you think most people have a clue what those "anatomies" might be?
Makes your question something of a red herring. Regardless of whether we settle on the Heying-Hilton-Wright Hypothesis (HHWH) or go with the biological definitions of Parker, Lehtonen, & Google/OED, people are still going to judge based on "proxy variables", on the basis of secondary traits that are part of the quite erroneous "family resemblances" conception of the sexes.
The issue is whether the stipulative definition of HHWH is "essentially correct about what makes a mammal either female or male". Apart from the many problems caused by leaving most sexually-reproducing (anisogamic) species out in the cold with it. But despite Damion's claims to the contrary, that is precisely what it is:
https://en.wikipedia.org/wiki/Stipulative_definitionA stipulative definition is a type of definition in which a new or currently existing term is given a new specific meaning for the purposes of argument or discussion in a given context. When the term already exists, this definition may, but does not necessarily, contradict the dictionary (lexical) definition of the term. Because of this, a stipulative definition cannot be "correct" or "incorrect"; it can only differ from other definitions, but it can be useful for its intended purpose.
And the fact of the matter is that the definitions of the HHWH flatly contradict the "dictionary definitions" that Google uses (Oxford) since neither they nor any other source says anything at all about "past, present, or future functionality":

Whether such definitions are, in effect, used by some biologists, it is still the case that HHW are "giving a new specific meaning (for 'male' and 'female') for the purposes of argument or discussion in a given context".
Seems to be some urgent necessity for such stipulative definitions from the biological community - too many are pussy-footing around calling a spade a shovel. Which only gives free rein to the charlatans and the grifters, to the deluded and the political opportunists, the result being a myriad of definitions for the terms in question such that they are all now more or less meaningless - and therefore quite useless.
Hilton and Company may well be commended for at least taking the bull by the horns, for drawing a line in the sand, even if I think their solution is very badly flawed from the get-go.