There are girls who are required for religious and cultural reasons to have a female only space. If that does not exist, they do not get to take part.
I don't know how clearer to be, I'm sorry.
As everyone has pointed out, scouting was forced to accept female members as they could not recruit enough boys, and more importantly, enough male leaders. They didn't magically agree to accept girls in some sort of grandiose sweep towards equality. They were forced to, to survive.
Girl Guiding is entirely, completely, 100% different.
Many of the leaders are involved because they believe that an all female space to try some of these activities (the same activities as the boys) is valuable.
I have no particular issue as to whether scouting accepts girls or not, tbh. I understand their rationale for doing so (and as I said, opened a group where none existed. There still wouldn't be one there today if two women had not bothered) and I understand the necessity. As an all boy's organization, numbers were dwindling, and a huge change and re-branding was necessary.
Girl Guiding would lose leaders and members if it became co-ed. There is in impetus to become co-ed. It would be detrimental to the values of the organization, and would result in a loss of membership. This loss would not be made up by hordes of boy-children and their daddies rushing to join, unlike the mummies and girl children that moved to scouting.
It will not happen.
It should not happen.
I can see no positive reason for suggesting such a move. At all.
In other news, one of my dad's was in two rainbow units (we moved). The first was brilliant. The second was shite. It's all about the volunteers, folks. But you can't criticize unless you are prepared to put out yourself. 