BingBong. This is the nub of the issue.
Legally, you’re on a hiding to nothing trying to argue that trans shouldn’t be a thing at all. The precedent at ECHR level is strong, it’s based on the right to privacy/family life (among others) and it’s well established over a number of cases. However, that right isn’t absolute. Most rights can be restricted for the purposes of crime prevention/public order/safety etc or even just to balance against conflicting rights of others. It’s just that this wasn’t argued in eg the Nicot case because it wasn’t relevant on the facts and the French State had no reason to bring it up as it wouldn’t have helped their case on the facts. These carve outs are there, though, and they give governments the discretion to design their systems in such a way as to permit these interests to be protected. So the single sex exemptions, and exceptions to the right not to be “outed“ as trans (re the ability of eg Police/employers/authorities to inquire about previous names etc for specific purposes such as DBS checks, manhunts, references — as discussed on here frequently) absolutely could be justified in terms of international best practice. It’s just that the trans lobby are the only ones bringing cases at this level, and they obviously won’t bring up these carve outs.
I suspect there is also an imbalance in that there are perhaps fewer female born transpeople seeking entry to traditionally male spaces than vice versa. In the Guides example it is obvious why...the Scouts already accept girls so no issue. Prisons — for safety reasons I assume no transmen have ever sought access ? Masons, Golf Clubs, the CatholicnPriesthood — I am unclear but perhaps there just isn’t the same champing at the bit to get in (or they just know they’d be refused and the SS exemption upheld).
It’s another example of the lopsidedness of this in favour of natal males. Actually, the current operation of the system , with the systemic failure to invoke and enforce the single sex exemptions (eg in official guidelines/use in practice by the police etc) is vulnerable to a challenge on the basis that it constitutes discrimination against natal females.