As best I can recall, males and females live in separate societies and the story begins with the two sides having just nuked the entire planet after centuries of conflict. A teenage boy is launched in an escape pod to the nearby moon which is habitable but not populated. Two young sisters also escape and land nearby. Because the sexes are separated, only high level government and military officials even know that the two genders can procreate. The three eventually bond and grow closer. Later another pod with two or three adult members of the female military is found and the commander (who knows about breeding) plots to get rid off the boy once he impregnates the girls and her subordinate.
Been looking for this one for a while.