It's been a while since I watched the Gareth Malone show, but I can't remember there being much emphasis on the fact that it was women only. It was all about having something to belong to, a time out etc etc. And as is apparent, men are involved anyway. The only real objection seems to be an artistic one.
To me tis is the point. I don't have an issue with serious choirs picking out the people who they need, including their sex. I also don't have a problem with single sex groups more generally.
But just saying "it's a female choir" doesn't cut it in all cases. It really depends on the reasons the choirs were set up. Was it to support military spouses and be a place for them to meet and interact, and so ended up being female more by default? That seems to be the case, even if since then they've naturally focused on women's voices. If that is the case, it makes sense for the group to really consider what their mission was meant to be and how they can now best fulfil that. If the point was always more about being female only, then that's different. But it seems that there is actually a lack of clarity about that which is often the case. It doesn't mean the question shouldn't be raised though.
There was a time when everything for military spouses was made up of women. It is difficult to imagine that everyone thinks that as male spouses began to appear they should have simply set up their own groups, and would have been wrong to ask to be included. Networks for spouses in the military in my experience are less about sex than other commonalities, and they do need a common social environment to work well, something that's actually less well established these days than it used to be in many military communities.
I also think a lot of women here would be pretty pissed off if the automatic response to any women wanting to join a group that has traditionally been all men was simply, nope make up your own group, rather than some real consideration of the purpose of the group.