Why was there a distinction made for women captives?

Question

In Numbers 31, God tells the Israelites to wipe out the Midianites (and incidentally, Balaam) by killing every male. But they spared certain women (v17-18):

Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him. But all the women children, that have not known man by lying with him, keep alive for yourselves.

Why did they spare these women (and not men who had not known women intimately)? And would this apply to women who may have been raped? How would they have told the difference, especially in a culture that may have been promiscuous?

Answer 1

In Chapter 25 we see that the Moabite women seduced some Israelites into temple prostitution. Therefore when God commanded Israel to raze the people of Midian he wanted the armies to kill the woman also and not take them as spoils of war. They did not do it at first but kept all the woman as booty. God was angry with them for that and then he makes it clear they can only spare virgins.

The idea seems to be that they can only take as plunder what had no threat of harm to Israel and no possibility of corruption from the Baal Peor worship.

Baal-peor.—Lascivious rites were widely spread and prevalent in Babylonia and Syria. See Knobel. [Also article Baal in Smith’s Bible Dictionary.—A. G.]. It was Baal, especially as he was worshipped at Peor, with lustful practice (hence Baal is sometimes called Peor). Beth-Peor, Deut. 3:29; 4:46. “He was a Moabitish Priapus, in honor of whom virgins and women prostituted themselves. As the god of war he was called Chemosh,” (Commentary on Numbers, Lange p167)

Naturally unless the woman were still virgins they may have already been involved in temple prostitution which was the original sin that they were being judged for.

Answer 2

The Talmud (Bavli, Y'vamos 60:2) deduces from the wording of the passage that the ones killed were not the women who had been intimate with men but those who physically could be, which is those who had reached their third birthday. (This is also cited by Rashi on verse 17.)

Answer 3

It's not difficult... They fought and after the enemy surrendered, all the enemy men - mostly the soldiers - were killed. Then the women with their children were taken to the camp. Here all the male children were clubbed to death. Then all the virgins - both adults and girl-children - were spared, while the non-virgins among the women were killed. Finally these virgins were forcefully married to men among their captures - assumingly so they would bare children who would swell the ranks of Moses' army.

It wasn't so much a question about finding the sexually pure, but to find women to bare children. And as the law demanded that one had to marry virgins - and further more that unmarried women who had "whored in their father's house" were put to death - all non-virgin females were killed.