I have a fairly quiet boy (well not at home, obviously), he goes to art club with his group of female friends after school. He also enjoys beavers, which happens to be just boys, and it has encouraged him to play football more, get involved in loud games and he has fun. I don't think that it has to be boy only, but I do think that it helps him to relate to other boys, if he had a free choice he might have chosen rainbows but he learns new skills and is challenged at Beavers.
My dd both enjoy the girl space at brownies and guides. One of them had a bad experience with a group of boys (in lower juniors) who were lying in wait for her, catching and kissing her despite her telling them not to over some months, the school did little to intervene and it has had a lasting impact (obviously we acted when we became aware). She still doesn't want to be in the same social space as boys much, she is for some clubs but mainly self-selects more girl dominated activities, although she is quite outdoorsy and not at all princessy. Brownies is also quite outdoorsy and she feels safe in that environment.
Ok you might say 'my son isn't like that/ wouldn't do that, he would only want to come for the craft and gentle activities', but firstly some of the boys involved weren't the big, bold footie players, they might also prefer a more girl dominated environment, but where would dd go then? Secondly it isn't just what the boy might want - there are other factors, what if beavers clashed with karate, so his parents thought he could go to rainbows instead? Maybe it would be easier for me to send ds to rainbows because they are all on the same night so I wouldn't have to have two nights of scouting/ guiding. There could be many reasons why a parent might choose guiding over scouts, not just because it suits their dc more. You can't screen 8yr old boys to determine whether they are likely to be wanting to kiss the girls or not.
I wouldn't mind either way if scouting was just for boys or mixed but dd needs that safe space. She would leave if some of the group of boys joined Brownies.