Association

People who define themselves in terms of their associations see nothing wrong with limiting their associations to people who are like them.  It is sort of a tribal thing.  Flocks of birds of a feather are actually more diverse because they do not, in fact, associate on the basis of appearance, but rather on the basis of common interests.  So, for example, a flock of black birds will be composed of crows and crackles and cowbirds and bobolinks and red wing blackbirds, all of whom like to forage in fields full of seeds and insects. A sudden noise will trigger the startle reflex and set them off.  So, to humans they look like a flock.

Humans, on the other hand, seem to segregate by gender.  Many resent the intrusion of females because they change the dynamics of the situation.  It is very difficult for a female to be recognized as an equal by males.  About the best one can hope for is to be ignored.