Its purely cultural, and apparently many people that are in sexist cultures, do not realize, evidently, that they are in a culture that is in fact sexist. AS such, any sexist view is taken as being normal and often not even noticed, like you don't see the air around you even if its everywhere and ubiquitous.
Ex: The college I was going to has large public restrooms/WC in the main building, in most cultures you look at the sign that says men or women as if its a given, but that was a restroom pure and simple, not a men only or a women only. In some cultures this would not be business as usual but something unusual (these find having bathrooms for men only, and for women only, "normal", in their on mind, that's the way its always been, its normal), and in other more conservative/archaic cultures where men and women must ride in separate elevators (Saudi Arabia, at least in the 80s) and all sorts of crazy sexist shite this would be a blasphemous outrage.
Someone used to such an environment(large public restroom, period), might find it, special, to designate bathrooms by women or men, the same way one might find it peculiar to have a little man sign to indicate that this is a men only cafeteria, or a little women sign in front of a staircase to indicate that this is the women's staircase.
~Of course you need to separate cafeteria, two separate stair cases, separate elevators, even in shopping malls, you need an escalator for men and another one for women, what? You mean that in your shopping malls men and women go on the same automated staircase escalators? OMG, how decadent! How come women are not raped? Btw, why aren't you carrying clubs and wearing animals skin?~
Note that someone that lived in a culture where there's an escalator for women and one for men, would probably think its normal and not even think about it, not realize how utterly absurd it is.
Some time ago I went into a small restaurant where there was two small bathrooms, one for men, the other for women.

Its logistically retarded, my GF went to the bathroom but the womens bathroom was occupied, she had to wait (in theory) while a bathroom was not being used, retarded, there was 2 bathrooms and two humans that needed but it was sub-optimal use, then I needed to go but there was a human that happen to be a man so I had to wait even if there was a bathroom available and not being used. I guess most people dont even notice this, but for me its freaking annoying and stupid. Its mathematically sub optimal and primitive. (I should have been born in the 23rd century).