There isn't anything that says that the Avatar incarnation has to switch genders every new turn; in fact that's directly contradicted by Roku and Aang both being male. It just happens to be that, barring the very rare genetic defects that pop up once in a long while, there are only two working sexes, so the odds are basically 50/50. And the creators decided that the Avatar following Aang is a girl from the Southern Water Tribe.
But even that had some element of fiat -- all that was really required was that the Avatar following Aang be Water Tribe, period; whether Northern, Southern, or Foggy Swamp. It could have been any of them. The creators chose Southern.
But even that had some element of fiat -- all that was really required was that the Avatar following Aang be Water Tribe, period; whether Northern, Southern, or Foggy Swamp. It could have been any of them. The creators chose Southern.