Why are you so bothered by this though?
I don’t understand why this is such a hard concept to grasp. If I send any of my DDs to anywhere, I want to know who is there. If I send my DDs to a place that is only meant to have group A and instead it’s group A and B mixed together I would want to know why I was misled. For example if a party is for our school members only, but it turns out anyone from other schools can attend it changes the dynamic. I would want to know why and why I wasn’t informed.
At the pointy end of this, for the welfare of my DDs, if something is supposed to be single sex - female - and there is a male there this does two things. It changes the dynamic, which may negate the whole reason my DDs are attending in the first place; they want to be with just females. And second, it raises a safeguarding concern.
This is not a reflection on the nature, good or bad, of the male there. But it means my DDs might be placed in a situation where they are alone with a male they don’t know. The teachers/leaders might be placing children of both sexes in situations that may not be appropriate (without being nefarious), so the leaders need to know what they are going to do in that situation and be able to relay it to me. This protects everyone: my DDs, them, and the male.
But a lack of open communication of such a situation would lead me to think they haven’t even thought about such issues, don’t have safeguarding policy and/or would think I am not entitled to have any say about their approach to supervising my DDs. I will not be lied to or gaslit into thinking everything is fine in this scenario because experience tells me that these organisations do not place the feelings or welfare of mine or anyone else’s DDs at the forefront, but do the males.
It doesn’t matter how minor it is, how small a thing it is, how it may be perceived that the thing they’re doing in this mixed sex group doesn’t pose a threat as far as they can see, I still want to know.
If I could confidently think that the world has the back of my DDs to do what’s right, then maybe I wouldn’t be “so bothered” but it’s highly likely their needs will be put last.
And finally by getting worked up over what might be a small thing to some people I am setting an example to my DDs that if you’re promised something as a right, it’s taken away from you, and then people pretend it isn’t happening (“that’s not a boy, silly!”), then let’s stand together and make a fuss about it and not just accept it meekly and let these people get away with it.
So I imagine that’s why the OP is bothered.