Well if it is true that the different races can produce offspring with each other (thus proving they are indeed different races of the same species), then the fact that certain races show a lack in population and genders must mean there is some underlying genetic function at work preventing it, such as with male miqo'te. Otherwise a female miqo'te could reproduce with males of other species (or vice versa) to increase the male population. So, in a way, we have to assume with miqo'te that the Y chromosome doesn't pair well with the X, and only happens rarely, and underlying genetic fault within the race regardless of their partner's race.