[ebaysucks]

The most basic requirement for a healthy societies is reproduction, females are the bottleneck in reproduction so killing both males is least bad in the long run.

If a war was expected in the short run, kill one couple as to forego some reproduction in exchange for a soldier motivated to fight.

Killing the two females would result in two depressed soldiers, so I would only pick this option if I was losing the war badly and needed the two males as cannon fodder - essentially killing all four.