I feel myself softening towards little tiny boys who would like to join Rainbows (the 4 to 6 age group), as there are very few actual adverse consequences to the girls - but there may then be girls who are no longer be allowed to attend. (I have to say that I don't know whether that would be the case: would some religions or cultures have a problem with that for the youngest children?)
I knew a family with triplets whose little boy was devastated, 20-odd years ago, that he couldn't join Brownies with his sisters.
But my woolly liberal feelings aren't really the point here. Guides are legally permitted to be single-sex. As far as I can see, there is no legal right to be single-'gender' (and I do wonder, here, whether the women's colleges now permitting male self-ID women to join are equally bending the law by excluding males who don't ID as women).
Single-sex is clearly definable, and single-gender isn't.
There has to be a better way than making nonsense of the language and the law.