Because of religious reasons, because a woman has suffered trauma and all the other reasons GC people have already repeatedly stated.
Because having unisex spaces means penis in women only spaces.
That is just another way of saying, flashing/voyeurism or assault might happen occasionally but we are ok with those odds.
Because what happens when the guard goes on break, or isn't paying attention, is distracted, or just doesn't care.
Because what about the tiny hidden cameras you can get easily these days.
Because what about the times when a small, quiet, locked and private area is the last place you would want to be.
Because there is more safety in: Numbers of women, in women's only spaces even just with flimsy curtains.
Than: Male bodied people being allowed in women's spaces, regardless of the measures taken to ostensibly keep women safe.
Single sex spaces says that we are not ok with it happening at all, so penis doesn't belong in women only spaces
The above isn't an exhaustive list either. I'm sure there are other reasons that I could articulate if I sat and thought about it for, oh I don't now 2 seconds.
You say you're GC but are not understanding some foundational concepts apparently.