Could the differences in structures of singing groups be due to the places they are happening? Our folk club at college, all girls, was mixed, with more women, because of where it was, but when I came out into the great wide world, I found that clubs were in pubs, not always the most open to unaccompanied women, (the nearest was also notorious for dealings in certain substances and it wouldn't have fitted my job to be seen there). Choral groups, choirs and operatic societies meet in more controlled environments, so can be seen as safe.