My understanding is that Midianite males, as well as most Canaanite males, are not allowed to live in the Old Testament context of war because they cannot be conjoined to Israel through a reception of the dominant cultural priority of God's Will and Holiness as expressed in Israel's moral law.
In the context of the Old Testament, Midianite males would only penetrate INTO Israel's dominance, and would in time attempt to displace Israel's dominance. They would not become incorporated. As with males, this also applies to Midianite women who have "already known a man," and they carry within themselves the corruption (from the assumed superiority of their gods) and the promotion of their cultural corruption through their filial relationships. Virgin Midianite women, on the other hand, are in a different position: They can 'receive' Israel's spiritual dominance and give birth to children in Israel without the taint of previous totalizing social acculturation.
Sam, think about how the following verses from Leviticus plays into the overall context of what we're talking about here:
Leviticus 22:24-26 New King James Version (NKJV)
24 ‘You shall not offer to the Lord what is bruised or crushed, or torn or cut; nor shall you make any offering of them in your land. 25 Nor from a foreigner’s hand shall you offer any of these as the bread of your God, because their corruption is in them, and defects are in them. They shall not be accepted on your behalf.’”
It is only when these "foreigners" willingly and completely become assimilated to Israel's Law that they can start to be considered worthy of joining into any level of social commerce with the people of Israel.
Similarly, and as an example, it's not ok for people to come to the U.S. and bring with them ideologies of either total Communism or Radical Jihadism; neither of these ideologies can fit within a democratically oriented republic. In fact, it would be foolish to think Communists or Radicals could be assimilated into our culture. It's just not going to happen ... only a naive person would think such foreigners could be 'at peace' with any of us who support democracy. Only those who are willing to adopt democratic notions of social discourse can assimilate to what we expect in our culture and society.
Peace,
2PhiloVoid