Sharing critical resources

In ancient history, harems were often created because of scarce resources. Frequent war fare drastically reduced the number of men, hence women had less opportunity to find a partner -- or they needed to marry the brother of their former husband, to be able to keep on taking care of their family. In agricultural societies with a high death rate among babies, the labor-intense activities were better spread among many children. And of course, rich man were able to attract women that would never have access to the luxury and education the wealthy family could offer them. Today, of course, these arguments are not valid any longer -- except for wealthy men. But today, career women face different scarce resources: they do not want to devote all there free time to a household and they have almost no time to maintain social networks other than the business networks they are part of. Spreading work over a broader group, increasing the family capital and being able to outsource labor-intense tasks, and enjoying a family of friends, could greatly increase the daily comfort of the family members.