How many times do we have to say it, though?
Transpeople who have transitioned should be accepted as their new gender, and use the facilities and spaces allocated to that gender. I don't think that anyone on here would object to that, although of course they should say if I am wrong in that assumption.
Non-transitioned people should use the facilities of the sex they were at birth, to prevent abuse of the system, and the chance of men claiming to be female in order to enter those facilities and be present in situations where natal women are vulnerable.
Self-ID should be the starting point for transition, if the self-identifying people want to be accepted into a different gender from their sex. If they do not wish to transition, they can wear what they like, adopt as many of the opposite sex/gender/whatever's norms as they wish, and ignore as many as they wish, just as people born into that gender do all the time, but they should not expect to be fully accepted as the opposite sex, because they have not transitioned and remain in their original body.
It is not transpeople who are a problem. It is not gender-fluid people who are a problem. It is not that people on this thread are transphobic or anti-trans.
It is people who have not committed to a change but want to access female spaces when they are still male who are the problem. For balance, I would extend that to women wanting access to male spaces, but as far as I can tell, this is likely to be less of an issue because they are less likely to be physically threatening to men.
In some cases this might result in awkward situations for the transwomen concerned, but with diplomacy and tact, they should be able to avoid sensitive areas where women are likely to feel vulnerable, and the awkwardness has to be weighed against the potential risk of having untransitioned men in single-sex female spaces.
ETA: If sex is binary for the vast majority of people, is this not a sensible basis on which be legislate? There is no need to be anything other than sensitive to the needs of those who do not fit this binary state, but the vast majority of the population should take priority.