Now actually this is where it really is simple and black and white (using your terms).
Any space that can be accessed by a male person is not then a single sex space for females. Regardless of the law about recognising their 'gender'.
It really is that simple. Either a space can be designated as single sex or they are never going to be. I haven't discussed women's clothes shops or WI meetings or any situation where women are not expecting to have privacy while they are in a vulnerable situation. Why have you mentioned these?
On a side note, since you did mention it, of course a store changing area should be properly secured and properly and clearly signed that it is mixed sex if it is allowing any male into the area, even with lockable doors.
Whether you, personally, believe that any male can be considered a female for the instance of toilets and whether toilets should be 'single sex' or not, makes no difference. Either a toilet is single sex or it is not. There is no nuance that should be allowed that will allow misunderstandings here. None.
And I don't believe that 'most people' believe that this is a 'grey situation' because the polls have all shown that women do not want any 'male' person with a penis in their toilets. Are you about to claim that 'most people' believe that toilets should not be single sex when they are described as being 'female' toilets? Because then you would be incorrect.