Appreciate all the above comments.
One other thought...
in terms of ethnic cleansing, many tribes and cultures would keep alive "breedable" females to promote their own tribal well being by producing offspring.
A woman who already had a child or who could possibly be pregnant with one, would be more trouble and more likely to raise that child with her own heritage in mind (see Moses). If the child was male he could grow up to be trouble. Why bother?
A woman impregnated while young with a tribal child would be more cooperative, She would be less likely to buck the status quo and also less likely to run away.
The save the virgins rule had a practical aspect from the point of warring tribes and domination of your enemies.