I think you just have to accept that it is the way it is.
If scouting had not decided to admit women then it would probably no longer exist in this country - for girls OR for boys. For a long period there were not enough boys wanting to join to make it viable - even more importantly there are not enough MEN willing to take on responsibility for running the units. You can't insist the organisation is "male only" when it is being run in many many cases by women. So it is nothing to do with sexism, and space to be boys - it is simply a matter of what needs to be done to keep the movement going.
Whereas guides has never had a problem with numbers, and has had no need to change their rules. They considered doing it anyway for the sake of equity, but there wasn't really much call for it, and there were good reasons not to (see Seeker's post(s)).
So in the end - you are just going to have to accept that it isn't a matter of sexism or equality, but simply a matter of the way things had to be.
I do find it interesting though that 99% of those who argue that the situation is wrong actually don't want girls to be allowed in their son's Beaver/Cub unit, rather than want to enrol their sons in Rainbows/Brownies. And thus don't appear to actually want equality and equal access, but just to stop girls from "changing the way things are" in Scouting.
FWIW - I am a Guiding leader and an assistant beaver leader - we actually have no girls in our beaver or cub units, but all the leaders are women as we can't find any men willing to commit the time. So really it is not a "male space" even then.