I always pondered about this, too. Does a Yeerk gender come from the host?
They call each others brothers and sisters, I think. So it implies two sexes. But there has been no mismix of a male Yeerk in the body of a female host, or vice versa, for us to confirm this. So in the end, it's kinda confusing, I agree.
Perhaps the Yeerks don't have sexes as we conceive, since they merge in threes and die in the process of reproduction.