... anyone wanna have a stab at it?
Perhaps, though the following is little more than informed speculation.
George E. Mendenhall, Frank Moore Cross, and others have emphasized the centrality of
kinship rights and covenants among the proto-Israelites. In fact, the traditions reflected in Deuteronomy were comparatively egalitarian, with strict rules for dealing with kin, servant and slave. In a period of arranged marriages and a religious insistence on sexual purity (i.e., virginity), only the girl child, of young virgin, would be seen as appropriate for integration into proto-Israelite society. To the extent that this is true, the exception made for virgins reflects, not a society intent on rape, but a society intent on taking into itself only those who could be treated 'equitably', i.e., in accordance with the law.
Of course, all this talk of slaughtering the evil Midianites is little more than jingoistic propagande, but even propaganda must pay homage to the realities and constraints of the society in question.