When you see toilet signs, they look more or less like this:
It is clear who can enter either of them.
Now, say you are designing the signs for showers and you want to distinguish between the ones for men and the ones for women. How would you do it?
Considering the women (and the men) have showers without clothes, the dress-shape-for-women is not valid. What can be painted to clearly indicate the difference without any kind of text?
I found this one but it is still a bit creepy:
User experience is more important than precision. Take the phone icon for example. Phones haven’t looked like this for a long time, but the icon is still very effective for communicating.
The two concepts you are trying to communicate are shower and woman. Luckily, there are common icons for both so it’s most communicative to just combine the icons. It doesn’t matter that the woman has clothes on….you are striving to be communicative rather than accurate.
The right icon avoids the clothing issue, but the left icon is more communicative because it combines two familiar icons.