No Girls Allowed, No Boys Either

"I'm afraid to say that the tendency to bond with one's own sex to the exclusion of the other is a childhood stage. It includes seeing one sex as worthy, strong and capable, and the other as weak, overly emotional, and less capable. It tends to show itself in day-to-day behaviors, where one sex is ignored, criticized, and micro-managed, insulted, and "punished" with dismissiveness, slander, betrayal, and various kinds of sabotage. The other is held in high esteem, high sympathy, and given access to resources and to leadership positions.

This developmental stage can be seen working itself out in elementary school-age children, more or less depending on the surrounding culture:
Cultures with science, math, language, the arts, and social instruction (simultaneously) as main focus tend to be successful in guiding children to grow past this developmental stage, but cultures without focus and cultures that are fixated on material gain and warmongering actually discourage children from maturing past this stage, keeping them in an unhealthy suspended state of childhood that they carry into adulthood.
If an individual grows beyond this stage due to outside influences, or of their own volition, they are usually singled out and excluded.