I can think of only two bands that have equal-or-fewer men: The Corrs (three sisters and their brother) and Skillet (two men, two woman, male primary vocalist). The Dresden Dolls kind of count, as a male/female duo, but Amanda Palmer was definitely fulfilling more of the "lead singer" role you discuss, though in a very unconventional way.

I've become wise to the diversity (or lack thereof) present in films/TV that you mention - the fact that having one or two women in an ensemble cast is enough. I'd never noticed it being present in music, but now I don't think I'll be able to stop.