The censor is a huge legal document. If sex doesn't matter on that then where does it matter? If it doesn't matter in the census then why should it matter on crime stats, health stats, poverty stats, income stats, etc etc etc. It's more like the fat end of the wedge than the thin end of the wedge in this instance. That seems really significant to me.
If there is a massive correlation between eg. race and educational outcomes then funding and action can be campaigned for. If there is a massive correlation between an ethnic group and a health condition eg. sickle cell anaemia then targeted funding and awareness raising campaigns and training medical professionals etc can take place.
The data matters and sex is as important and ethnicity or disability in that it is a protected characteristic the same as them in the Equality Act. The data informs policy, funding, investigations etc.
I don't want to hear in 5 years time that there has been an exponential rise in women convicted of sexually abusing minors if in fact what has happened is a rise in child abusers 'identifying as female'.
I don't want to hear that women are now better represented on the boards of top companies if what they mean is men who wear dresses on Thursdays and like plucking their eyebrows are being counted as women.
I feel the need to say etc a LOT because the implications and chains of consequences are huge.