In order to provide additional insight into this account, I am pasting the entire chapter from the Jewish Publications Society TORAH. After reading the account scroll to the bottom for alternative viewpoints:
**********************************************************************
Numbers Chapter 31
1The Lord spoke to Moses, saying, 2Avenge the Israelite people on the Midianites; then you shall be gathered to your kin. 3Moses spoke to the people, saying, Let men be picked out from among you for a campaign, and let them fall upon Midian to wreak the Lords vengeance on Midian. 4You shall dispatch on the campaign a thousand from every one of the tribes of Israel. 5So a thousand from each tribe were furnished from the divisions of Israel, twelve thousand picked for the campaign. 6Moses dispatched them on the campaign, a thousand from each tribe, with Phinehas son of Eleazar serving as a priest on the campaign, equipped with the sacred utensils ( Perhaps the Urim; cf. 27.21.) and the trumpets for sounding the blasts. 7They took the field against Midian, as the Lord had commanded Moses, and slew every male. 8Along with their other victims, they slew the kings of Midian: Evi, Rekem, Zur, Hur, and Reba, the five kings of Midian. They also put Balaam son of Beor to the sword. 9The Israelites took the women and children of the Midianites captive, and seized as booty all their beasts, all their herds, and all their wealth. 10And they destroyed by fire all the towns in which they were settled, and their encampments. 11They gathered all the spoil and all the booty, man and beast, 12and they brought the captives, the booty, and the spoil to Moses, Eleazar the priest, and the whole Israelite community, at the camp in the steppes of Moab, at the Jordan near Jericho. 13Moses, Eleazar the priest, and all the chieftains of the community came out to meet them outside the camp. 14Moses became angry with the commanders of the army, the officers of thousands and the officers of hundreds, who had come back from the military campaign. 15Moses said to them, You have spared every female! 16Yet they are the very ones who, at the bidding of Balaam, induced ( Meaning of Heb. hayu ... limsor uncertain.) the Israelites to trespass against the Lord in the matter of Peor, so that the Lords community was struck by the plague. 17Now, therefore, slay every male among the children, and slay also every woman who has known a man carnally; 18but spare every young woman who has not had carnal relations with a man. 19You shall then stay outside the camp seven days; every one among you or among your captives who has slain a person or touched a corpse shall cleanse himself on the third and seventh days. 20You shall also cleanse every cloth, every article of skin, everything made of goats hair, and every object of wood. 21Eleazar the priest said to the troops who had taken part in the fighting, This is the ritual law that the Lord has enjoined upon Moses: 22Gold and silver, copper, iron, tin, and lead23 any article that can withstand fire these you shall pass through fire and they shall be clean, except that they must be cleansed with water of lustration; and anything that cannot withstand fire you must pass through water. 24On the seventh day you shall wash your clothes and be clean, and after that you may enter the camp. 25The Lord said to Moses: 26You and Eleazar the priest and the family heads of the community take an inventory of the booty that was captured, man and beast, 27and divide the booty equally between the combatants who engaged in the campaign and the rest of the community. 28You shall exact a levy for the Lord: in the case of the warriors who engaged in the campaign, one item in five hundred, of persons, oxen, asses, and sheep, 29shall be taken from their half-share and given to Eleazar the priest as a contribution to the Lord; 30and from the half-share of the other Israelites you shall withhold one in every fifty human beings as well as cattle, asses, and sheep all the animals and give them to the Levites, who attend to the duties of the Lords Tabernacle. 31Moses and Eleazar the priest did as the Lord commanded Moses. 32The amount of booty, other than the spoil that the troops had plundered, came to 675,000 sheep, 3372,000 head of cattle, 3461,000 asses, 35and a total of 32,000 human beings, namely, the women who had not had carnal relations. 36Thus, the half-share of those who had engaged in the campaign [was as follows]: The number of sheep was 337,500, 37and the Lords levy from the sheep was 675; 38the cattle came to 36,000, from which the Lords levy was 72; 39the asses came to 30,500, from which the Lords levy was 61. 40And the number of human beings was 16,000, from which the Lords levy was 32. 41Moses gave the contributions levied for the Lord to Eleazar the priest, as the Lord had commanded Moses. 42As for the half-share of the other Israelites, which Moses withdrew from the men who had taken the field, 43that half-share of the community consisted of 337,500 sheep, 4436,000 head of cattle, 4530,500 asses, 46and 16,000 human beings. 47From this half-share of the Israelites, Moses withheld one in every fifty humans and animals; and he gave them to the Levites, who attended to the duties of the Lords Tabernacle, as the Lord had commanded Moses. 48The commanders of the troop divisions, the officers of thousands and the officers of hundreds, approached Moses. 49They said to Moses, Your servants have made a check of the warriors in our charge, and not one of us is missing. 50So we have brought as an offering to the Lord such articles of gold as each of us came upon: armlets, bracelets, signet rings, earrings, and pendants, ( See note at Exod. 35.22.) that expiation may be made for our persons before the Lord. 51Moses and Eleazar the priest accepted the gold from them, all kinds of wrought articles. 52All the gold that was offered by the officers of thousands and the officers of hundreds as a contribution to the Lord came to 16,750 shekels. 53 But in the ranks, everyone kept his booty for himself. 54So Moses and Eleazar the priest accepted the gold from the officers of thousands and the officers of hundreds and brought it to the Tent of Meeting, as a reminder in behalf of the Israelites before the Lord.
Click here to go to Chapter 32
Copyright (c) 1985, Jewish Publication Society. All Rights Reserved.
**********************************************************************
Notice that the age of the captive girls is NOT mentioned. The hebrew rendered here simply translates as "young girls". Since the age cannot be determined, and the command was to ensure they were virgins, and we know the Isrealites practiced slavery, is it not possible the captives were simply distributed as "slave girls" amongst the warrior's households?
There are other accounts regarding slave girls in the scriptures. Consider the account of the slave girl who told her master to visit the prophet in isreal and bathe in the jordan to cure leprosy. There a young girl is also described, who was in servitude.
Nowhere in the account do we find a specific purpose outlined for the young girls. Therefore, it may be extreme to assume a sexual purpose existed, unless there is further evidence to suggest it.
Just my two cents!
