Sometime during my lifetime, when I wasn't paying attention, the word "Sex" got replaced with the word "Gender". I think this was an achievement of (American) prudery in combination with a banal social constructionism - the view that everything is social, nothing is natural. Since I just had to explain social constructionism, my guess is that prudery did the bulk of the work.
This loss of an important distinction impoverishes the way we think
Human beings, like other animals, are born sexed and in 99% of cases, they are unambiguously sexed as male or female. That's why it's reasonable to ask someone to tick a box for their Sex.
Hermaphroditism is rare and, at most times and places, a misfortune: see, for example, Michel Foucault's edition of the Memoirs of Herculine Barbin or the Argentinian film XXY.
Gender is a social construction: one is not born a woman, one becomes one. But Simone de Beauvoir should have added: normally, only if one has already been assigned to the female sex.
Cultures construct genders on the basis of anatomy. They do it in different ways, rarely with unqualified success, generally making mistakes along the way. Your Mum and Dad, they fuck you up ...
The frequency of these mistakes is the main reason why it is unreasonable to ask someone to tick a box for their Gender. People really need an essay to do the question justice.
Most males and most females are gendered as a mixture of what are regarded as masculine and feminine traits. In some cases - and it's much more common than hermaphroditism - males end up basically gendered as female and females end up gendered as male. Social construction is not a smoothly functioning, bug-free program, even in the most sex-role stereotyping societies.
At the same time, it has always seemed to me implausible to think that males and females differ at birth only in sexual anatomy. It's entirely plausible that the ways they do differ are not very important. But suppose they are. Then it's still the case that natural differences do not determine an appropriate social and cultural response. If males are naturally good at X and bad at Y this does not settle the question: Should they be encouraged to concentrate on X and forget about Y or, alternatively, allowed to get on with X but made to take extra classes in Y? Plug in anything you like for "X" and "Y".
If you re-open the distinction between sex and gender you can consider options currently occluded. For example, it may well be important to balance up the sexes in an organisation's work force. But if they are all gendered the same way - masculine men and masculine women, for example - the organisation may still be missing things from which it could benefit.