If it's a strictly girl-only environment, then adult women are no longer girls and thus can't be there either. If Brownies is only for girls aged 7-10, then an adult woman wouldn't qualify to join, any more than a boy or man would. Except, they aren't joining - they're enabling in the capacity of kind, responsible, DBS-checked adults - just like Dave.
Comparisons with men-only clubs are silly, because men (same as women at places like the WI) don't need adult supervision at their clubs - and they likely have female staff serving them anyway.
Surely the idea of girl-only is for those being served by and benefiting from the club (so no boys playing with them and taking over or embarrassing them) - not those doing the serving. I'm sure the adult female leaders aren't desperate to run under parachutes, squeal whilst playing energetic games and make glittery unicorn crafts either.
As for ‘warning’ parents about a man helping out, you know exactly what that’s going to lead to. People will come to their own conclusions and the fact that they had to be ‘warned’ will already condemn him as a pervert in their eyes.
There are so many men out there (many of them fathers) who couldn't care less about nurturing and serving children and I think it's so sad that, when they do (like Dave), some women are outraged, assume they're up to no good and want them banished.
I think that, reading a lot of the comments here, there are a lot of women who don’t want men to be a proper part of society. Just because most violence and child abuse is perpetrated by men does not mean that most men are violent and/or child abusers. What’s the solution, then? Send all the men to live on a massive island and just send a courier when society needs some sperm – and then, the children that ‘sadly’ turn out to be boys get sent there as well at 18?
Just remember, those of you who refuse to let you go out for an evening with friends because your husbands don’t want to look after their own children: he isn’t doing it because he’s lazy, selfish or controlling at all – he’s doing it to prove to you that he wants them to be kept safe, and that he, as a possessor of XY chromosomes, through no fault of his own is simply not capable of that. 
From my personal experience as a child I think any boy/man actively seeking out a variety of ways to be with children is dodgy as fuck. I have a Dad, husband, male friends, son. I don't hate men at all, just noticed that the decent ones seem to instinctively know to avoid putting others in uncomfortable situations.
I presume your husband didn’t have any say in your having a child together, then? That’s a guaranteed way to get to spend time with children – MUST be a pervert, 100%. If he wasn’t he never would have put you in that dangerous position of risk 