Maybe my head has been in the sand or the clouds but... why did the Boy Scouts start admitting girls? If we agree that girls should have girl-only organizations

Maybe my head has been in the sand or the clouds but... why did the Boy Scouts start admitting girls? If we agree that girls should have girl-only organizations to support, mentor, and grow their minds etc why can we not agree to the same for boys? It seems in accepting girls, the Scouts have erased a boys-only space and forced them to share it with girls for...what? Where is the gain?