“Because of the protracted childcare in humans, a premium has been placed on post-partum parental investment, so female mate choice is governed by the desire to obtain resources from long-term mates, an objective that conflicts with man’s desire for multiple partners.”
These forces put man in a position of wanting it both ways – stable long term partners associated with producing heirs and secondary partners used outside the accepted relationship. Indeed, Scheidel asserts that the first monogamous societies (Greece and Rome) were socially imposed. By this he means there was social pressure to monogamous based on the increasing importance of cooperation among coalitions (peer groups), needed to deal with external challenges. This pressure led to a moral standard that monogamy should be the only acceptable marriage practice.