Then your claim that these were single sex spaces is bull ◊◊◊◊.
Just because it is illegal to keep males out does not imply that the spaces were not single sex
de facto, as the result of cultural inertia combined with the fact that the vast majority of males would be (rightly) hesitant to expose themselves to a roomful of women and girls, on grounds of personal sexual modesty and on ethical grounds.
Everyone going there was aware that they were gender segregated, not sex, insomuch as they are familiar with the language and their representatives' legislation.
I've no idea why you'd expect ordinary spa-goers to be familiar with cutting-edge civil rights jurisprudence. Even trained California lawyers could not know in advance whether the amendments to the civil rights code would be interpreted as banning sex segregation in favor of gender segregation, although they should probably have known that the ACLU was fighting for this particular interpretation if they spent time around those sorts of activists.
We know there WI spa had a list on that very day of transgender patrons, of which Merager was one.
Even if they did maintain a list (source?) it wasn't tacked up on a public bulletin board.
What was the transgender policy at that time?
Wi Spa immediately issued a statement that they strive to avoid running afoul of state antidiscrimination law, and we can probably take them at their word on this, although that there was also an email instructing managers on duty
not to admit Merager in particular.
What was disclosed to patrons?
I'm fairly confident patrons were not told in advance to expect to find bepenised people on the women's floor, although California civil rights lawyers would have known in advance that this was always a live possibility.
Oh, so you want to return to the good old days, when people knew their place?
In former times, males knew not to expose themselves to minor girls (even at the spa) and I've already said that I would welcome a return to that norm being broadly understood and socially enforced. I can think of some other things we did better around the turn of the millennium, if you are interested, but it hurts my old head to call that the "good old days."
The argument was that this was not a male specific argument.
Any norm against exhibitionism is going to overwhelmingly constrain male behavior, that's just a fact you have to take into account.