Farkel mentioned Numbers 31 and wrote:
"What possible reason would Moses (through the wishes of Jehovah) want to do this? So the Israelite men could have tiny little sex slaves in their possession, that's why. Otherwise, Moses wouldn't have been so crystal clear just what kind of little girls to save and what kind to slaughter."
The reasoning (not that I agree) was that, since it was Midianite women who played the major role in seducing Israelite men, they were to be killed, along with men (who would of course be engaged in battle) and boys (who might eventually want to avenge the loss of their parents).
Moses was ticked off because, initially, the Israelite men spared the Midianite women, who were considered as much a future danger (through seduction of apparently spirtually spineless Israelite men) as Midianite men (via war) would be. The younger virgin women, on the other hand, were evidently more malleable and easily subjugated.