Imagine a far away land, a long time ago, in which the plains were vast and across them came flowing marauding hordes on horseback. The people on horseback did not come in peace. The people on horseback had come for conquest. The people on horseback were men.
The local people, being fond of their lives and their lifestyles, their farms and their families, fought back, but to little avail. The local men, mostly, were slaughtered. So too were the children. Many of the women were raped. Some of the invading men stayed in the new land, and some went back to the lands and families that they had left behind. Children born of rape in the new landscape; children born of love—or at least not rape—in the old. Two different sets of women, mothers to all of the children. One set of men, who fathered children in both places.
Or perhaps these men who came flying across the vast plains on horseback came from a land which held little for them to return to. Perhaps, indeed, that land had a tradition of extreme polygyny1, in which a very few, very powerful men, controlled and monopolized the majority of women, leaving most men without reproductive options.