Why did bathrooms get segregated by gender in the first place?

http://www.pinknews.co.uk/2016/06/01/why-did-bathrooms-get-segregated-by-gender-in-the-first-place/

Pink News/Terry Kogan, University of Utah  06/01/2016

"Amid rows about transgender people in bathrooms, Utah law professor Terry Kogan asks... how did public bathrooms get to be separated by sex in the first place?
For years, transgender rights activists have argued for their right to use the public restroom that aligns with their gender identity. In recent weeks, this campaign has come to a head.
In March, North Carolina enacted a law requiring that people be allowed to use only the public restroom that corresponds to the sex on their birth certificates."

An interesting read.  But I'd add that anytime prurience rears its head in western civilization, you can usually find St. Augustine if you look hard enough. 

Because both the political left and right send the message that women should be afraid of men.

Now of course bathroom segregation doesn't actually increase safety any. But it makes women feel safer. Same as