It's simply a more natural impulse among men. Not all men, obviously, and among those who feel it, not all the time. As in, it's more a thing that occurs to them at some point in their lives. But I think it’s certainly not an abnormal development for a man to have the feeling or urge (the drive to share a mate/be cuckolded by Her) even if it isn't all that common a practice yet. But I do think there's a chance that may change, and that people may look at it very differently in the near future.