I do understand where you are coming from and I probably should have elaborated a little bit more. Of course there is nothing wrong with a girls hanging out with boys and visa versa, but me using the Sandlot 1 and 2 as a reference was to show how the luster of the 1 film was the ability for men to relate to a film because of their childhood experiences with their other guy friends. Then we see with the 2nd film introducing a cast of 3 girls with the boys which diluted that luster that the first film was able to give to the audience. Now, I get it, and I am not revolving my philosophy around films. This was just supposed to be a small anecdote for comedic purposes. Anyway, I think you can see where I am getting at. The boy scouts offers that quality where it is a group of boys that can be together and share experiences with each other that are unique and special than sharing it with the opposite sex. Like wise for the girl scouts.
For your point about boys being friends with girls and girls being friends with boys, that is totally fine and normal, but as for organizations, sports teams, and etc. I believe it should be gender exclusive so that those two genders are able to build off of the same sex to grow and cultivate unique experiences that are different from different sex interactions.