Problems.
Laws exist but people don't just go along with them, they have to be enforced. eg illegal to sack a woman because she's pregnant, but happens to thousands of women every year.
If the laws aren't upheld spontaneously, then the recourse is court. Very very expensive. And stressful etc.
We have individual women having to take orgs including the NHS to court to get the law enforced, and the civil service etc are fighting them. When what they have done is illegal in law.
So that's a bit of a problem.
The other point is that this is the same situation as girl guides, really.
GG it turned out had changed from a single sex org to a single gender org.
They claimed they had always been single gender not single sex (really? A B-P set it up as single gender not single sex in 1910? We are supposed to believe that????!!).
Charity rules - they have a single sex exemption due to girls needing something separate from boys in these circs due to female oppression etc etc.
By changing to single gender they are:
Discriminating against some male children, as some are acceptable but not others
Discriminating against some female children who have a gender id that is not 'girl'. (Not sure what they're doing in practice. Initially they were going to suggest to non binary girls that they look elsewhere. I suspect they have no clear policies).
Discriminating against girls who are not allowed to go to mixed sex activities (often due to religion, could be other reasons).
Safeguarding. Groups like scouts are mixed and their safeguarding takes this into account. Guides said there was no need to review their (single sex) policies as trans girls are girls...??!! Camping changing everything no need to review. That's just s statement I couldn't quite believe. GG rangers are 14 to 18... I mean wtf tbh