I know they are less common now than they were but watching Carols from Kings has made me think about this.
When I was a child I was in a girls-only CofE choir, so SSA. It was unusual at the time. Children who sing in good quality church choirs (cathedrals etc) get a really good grounding for a career in music and I'm assuming still get scholarships to schools and colleges. Yet some are still only open to boys and men.
Where I live as an adult there is a big church with two very good choirs: boys and men, and girls and the same men. I believe the younger men get a small stipend (beer money for the student types) and of course both the upper voices and tenors could be sung by women. Again it's an intensive singing training that young women can't get.
My understanding is that girls only sing very solely differently to boys due to socialisation before puberty (girls learn to sing like other girls). And I imagine that some girls sing in a more "boy" manner.
So how is this still possible? Especially for school/college age, when it's not a club of like minded adults?