Because the men have almost all killed one another off, leaving mostly those who managed to stay out of the fight? Who is supposed to teach the kids skills to rebuild civilization if who's left are wholly occupied with reproducing? Will you kill the infant boys too?
Regrettably, yes. But it is, you know, a sacrifice required for the future of the human race. I hasten to add that since each man will be required to do prodigious... service along these lines, the women will have to be selected for their sexual characteristics which will have to be of a highly stimulating nature.