non-trans men could be excluded from a service on the “biological sex” exemption , even though the service was including some “birth males” - trans women - without necessarily unlawfully discriminating against (non trans) men.
So, a couple of points here:
The suggestion of excluding a particularly trans man, which I agree has lead to unhelpful misreadings, was not that the trans man would thereby be included in male provisions, but that they may reasonably be asked to use unisex provisions should they exist.
Key points here being that (1) the exception is an allowed exclusion from natal sex provisions not an inclusion in opposite sex provisions (2) is dependant on the availability of additional unisex provisions, and (3) the exclusion is to achieve the legimate aim of the single sex provision, in this case providing a space where women feel safe from men.
Emphasing here: the justification of the exclusion is the same as the legitimate aim that justifies the space: the collective needs of the other women in the space not the individual need of the trans man.
So the equivalent scenario for a trans woman is not that he is included in the women's provisions as a better fit for his own needs (which after all is likely to undermine the legitimate purpose for other women not to mention losing the sex based exemption) , it would be that he were excluded from the male provisions in favour of unisex because his presence was upsetting to other men.
And just as for the trans man, if there were no unisex provision, he would remain with the facilities of his sex.
You are also right that the other concern might be that a provider, in allowing trans women in a facility, wasn’t providing appropriate single sex facilities - but the contexts where this is required are limited (eg schools, workplaces)
I think you have also misunderstood this. Regardless of where single sex facilities are mandated, any situation that only provides mixed sex provisions could be at risk of indirectly discriminating against one sex if it could be shown that having only mixed sex provisions disadvantages one sex over the other.