Thing is, we shouldn't be treating people differently based on either their sex or 'gender', except where there is a valid reason/need for it, and all the valid reasons I can see relate to sex!
Why/where would we treat people differently based on how masculine/feminine they look, or say that they feel? Isn't that just sexism - which we've been trying to get rid of?
The exceptions in the Equality Act, though they may not cover quite everything, give a pretty good steer on the types of reasons we might need to treat people differently based on their sex, such as:
- Differing health needs between males and females, different drug doses, different conditions or same conditions presenting differently, etc
- Physical differences (strength, speed, size) - relevant for sports in particular, clothing/shoe sizing potentially, safety in some cases
- Privacy and dignity - not wishing our intimate areas to be exposed to the opposite SEX (who have different bits), or them to expose theirs to us (hence separate showers, toilets, intimate care, changing rooms...)
- Issues relating to pregnancy, maternity, menstruation/gynaecological issues, and the much greater impact (on average) of having children on women compared to men
- Sexism - lower rates of female representation, discrimination against women, awards/schemes etc. that go primarily to men unless there is a female category/separate award...
- Male violence against women/girls - especially sexual violence, which is vanishingly rare in the opposite direction, domestic abuse (which is also heavily sex-biased), etc - which creates the need for women to be protected from men in certain situations, and separate female-only provision in accommodation, prisons, rape counselling, shelters, etc.
These are all based on SEX, I can't see that any of these are significantly impacted by how someone dresses, refers to themselves, or feels inside if they are not of the relevant SEX. (Some people may have biases against people trying to appear as the opposite sex, but that is a separate issue, as it's not due to any confusion about their actual sex!) And there is plenty of evidence that "identifying as" the opposite sex does not change these factors significantly - males will still be male-bodied, rates of male violence aren't lower (and appear to be potentially actually higher) in men who identify as women, and most women don't feel any more comfortable with intimate exposure (on either side) just because a man says he's a woman. So there is no good reason to extend adjustments made to address these issues to people who are not of the intended SEX, and indeed, in most cases, that will defeat the whole point of them, as they will no longer deliver the benefits they were designed to if they now include both sexes anyway!
But apart from these scenarios and related adjustments which are clearly based on SEX differences, what possible reasons/excuses are there to treat men and women differently (especially if it's based on appearance, behaviour or claimed feelings, i.e. stereotyping...) that aren't basically sexism?