You are missing the point. It is precisely the fact that parents don't even bother to articulate it, but act on it, that makes it rock-bottom basic to our society, unlike others.

Parents send their kids out into the world, and encourage them to do so. If a shy girl is reluctant to socialize with others of both genders, that  is considered a problem and parents and teachers will try to encourage her to do so. Our organisations are mixed gender, those that were closed to women were the object of feminist pressures to become so. 

Mennonites are a red herring, I am not talkineven those Mennonites mingle socially with unrelated others in their community, young people have social activities in order to meet their potential mate, etc. ., and I'll bet the girls are encouraged to do so. And I have never heard of Mennonites demanding their wife or daughter not be examined by a male doctor, etc. You seem to know little about them, actually.