It happens because over the last 10 years the likes of Stonewall have sown confusion amongst service providers and promoted the law as they wish it to be. That shouldn't be taken as consent from women or a situation that is unrectifiable.
and
Because a dna check isn't necessary. Checks of any kind have only become necessary because some have insisted on breaking the social convention.
These two statements are mutually exclusive to each other.
Statement (b) of this pair shows that you acknowledge that it was never against the law for people to use the services that align with their needs, In that, you are perfectly correct. Statement (a) demonstrates that ''social convention'' is something you perceive and believe to be true, but you accept is not entrenched in UK law. That is a system of belief and not legal certainty.
The consequences of your words:-
Stonewall never ''promoted the law they wish to be''. In fact they stated the law as it was, that there was no law that said that men could not enter women's spaces, and women could not use men's spaces. The deeming provision in the GRA in concert with the provisions of the Equality Act fully allow trans people to select the services that align with the legal sex and gender identity. According to Vera Baird, it wasn't even necessary to have a GRC to do so. What she did say is that those with a GRC are protected on the grounds of sex, and those without a GRC are protected on the grounds of gender reassignment. Reading the documents that were used by Baird and Field to guide the EqA through both houses, shows that this interpretation is indeed what parliament intended, and anything else is as in your own style ''magical thinking''.
As Naomi Cunningham herself has noted, the constraint on the perception of social convention is that exposure of genitals (and that's either way round) can be tantamount to sexual harassment. As
@icowden has told you, times aplenty, the law pertains to conduct not a system supposed by you for him not to be present there. Using the facilities is acceptable conduct. We don't need to provide a list examples of unacceptable conduct to expand upon that, but women do this all the time. I've often exited a cubicle only to unintentionally and unavoidably catch sight of a woman exposing her parts. Technically that could be argued as sexual harassment in the same way as ''cottaging'' used to be an arrestable offence (importunity) in the case of gay men.
Not closing the door while with your lower half is undressed is the breach of conduct, not mere presence.
In other countries different social cues apply. What you are claiming is that a Victorian sense of etiquette holds legal preponderance. It doesn't. Ian's reading of UK law is the more correct.