"I understand that you cannot disclose identities of any trans members. I would rather my daughter did not share a changing room or sleeping quarters. Can we discuss this?"
Let's imagine a scenario where there is a transgirl in a GG troop who passes so well that no-one other than the troop leaders are aware that they have a male body.
Sarah and Molly's parents are MNers and know that there is a potential issue. They are sharp elbowed enough to send this email, and seek - and receive - assurances that Sarah and Molly will share a tent with their friend Anna only at the forthcoming residential. They also request that Sarah and Molly don't share shower facilities with a transgirl, and this is agreed.
Anna's mum doesn't know about the policy, but is pleased that her daughter is sharing a tent with her friends and thinks no more about it. Because she doesn't make the request for Anna to shower with girls only, Anna is expected to share the washing facilities with a transgirl. This comes as a complete surprise to Anna, who is deeply uncomfortable with being naked in front of someone with a male body.
Lorraine's parents are similarly unaware of the policy, and would be horrified to learn that Lorraine is sharing a tent with someone with a male body. Had they known, they would have made clear that this was not acceptable, but they didn't so Lorraine just has to suck it up.
Nina's parents are Muslim and would withdraw Nina from GG rather than allow her to share a tent and washing facilities with a transgirl. When Nina gets home from camp and they find out what has happened, Nina is withdrawn with immediate effect. Her uncle is even more conservative, and makes Nina feel shamed for what happened.
Jane's parents knew about the policy but didn't want to make a fuss. It never occurred to them to complain, they just told Jane if she found herself in a tricky situation that she should try to excuse herself and try to shower later, for example. Jane is confused.
And so on, and so on....
In short, even allowing some girls to opt out of the "fully inclusive" policy doesn't help - those who don't know about it or who aren't willing to make a fuss tend to be some of the most vulnerable (parents with poor English, for example, won't understand the word salad gubbins that the GG put in that email). How anyone thinks that is acceptable is a mystery to me