Moses, Eleazar the priest, and all the leaders of the people went to meet them outside the camp. But Moses was furious with all the military commanders who had returned from the battle. "Why have you let all the women live?" he demanded. "These are the very ones who followed Balaam's advice and caused the people of Israel to rebel against the LORD at Mount Peor. They are the ones who caused the plague to strike the LORD's people. Now kill all the boys and all the women who have slept with a man. Only the young girls who are virgins may live; you may keep them for yourselves.
Numbers 31:7-18
You are actually leaving out a parge part of the scripture reference you pointed to. The full reference is as follows:
"
7 They attacked Midian just as the LORD had commanded Moses, and they killed all the men.
8 All five of the Midianite kings--Evi, Rekem, Zur, Hur, and Reba--died in the battle. They also killed Balaam son of Beor with the sword.
9 Then the Israelite army captured the Midianite women and children and seized their cattle and flocks and all their wealth as plunder.
10 They burned all the towns and villages where the Midianites had lived.
11 After they had gathered the plunder and captives, both people and animals,
12 they brought them all to Moses and Eleazar the priest, and to the whole community of Israel, which was camped on the plains of Moab beside the Jordan River, across from Jericho.
13 Moses, Eleazar the priest, and all the leaders of the people went to meet them outside the camp.
14 But Moses was furious with all the military commanders* who had returned from the battle.
15 "Why have you let all the women live?" he demanded.
16 "These are the very ones who followed Balaam's advice and caused the people of Israel to rebel against the LORD at Mount Peor. They are the ones who caused the plague to strike the LORD's people.
17 Now kill all the boys and all the women who have slept with a man.
18 Only the young girls who are virgins may live; you may keep them for yourselves." - Numbers 31: 7-18
The young girls were allowed to live as wives to the Israelites. Not to be kept as love slaves. This passage is just a short chapter that tells the story of how the Israelites conquered Canaan. God used the Israelites as a vessel to punish the Canaanites for their sin. The adult women were the ones responsible for, as the scripture states, the Israelite rebellion at mount Peor, and subsequently caused Gods judgement be sent onto the Israelites in the form of a plague. The young women were given the chance to live as wives of the Israelites as an act of mercy. They were essentially given a second chance to escape God's wrath on the condition that they would become wives of the Israelites.
According to Deuteronomy 22:25-29, the Bible actually gives a very clear call for the execution of any man found guilty of forcing another woman to lie with him.
