They're basically the females for all the races except humans and quarians... And as amusing as the conversation between the human and turian who argued over which race asari are more similar to is, it does not justify the lack of aliens of a female gender. No matter what justifications mass effect gives for it, I 100% believe that it's simply because bioware did not want the trouble of creating new character models, etc. And I feel like it hurts the world building put into the game quite a bit.