Presumably if he is gender fluid he has kept all his male equipment, vq.
In which case, if he went into the men’s changing rooms, the other men wouldn’t know he was having a ‘she day’ unless he went out of his way at to tell them.
On the other hand, if he went into a women’s changing room, the women wouldn’t know whether he was a gender fluid person having a ‘she-day’, or a masculine person pretending to be gender-fluid so they could look at women getting changed. I would feel rather intimidated if I was confronted in a changing room or toilets by someone who looked like a man, because how would I know that that day, they happened to feel like a woman?
I have the greatest sympathy with anyone who genuinely feels they have been born the wrong sex, and goes through the whole process of hormone therapy and/or surgery.
But I’m afraid I don’t get gender fluidity and changing one’s gender on a daily basis.