I've found some excellent vocal groups from parts of Asia not known for being especially Christian, but it seems that choral/a cappella music is very connected to Christianity there too.
I believe church attendance is getting more gendered too, with a shortage of men.
I'm in Houston. I have seen nice a cappella from the Philippines but that must not be what you have in mind, large Christian population there. Where were yours from? From what I've seen of African church singing, they were always mostly female.
I've found some excellent vocal groups from parts of Asia not known for being especially Christian, but it seems that choral/a cappella music is very connected to Christianity there too.
I believe church attendance is getting more gendered too, with a shortage of men.