Some biologists don't think it is:Pairing Up for Life
Many species of birds are known to pair up for one breeding season or for life. The main reason is that their newly hatched chicks need the constant labor of two parents to keep them fed and alive. But DNA tests show that most of them practice only social monogamy, not sexual monogamy. "Adultery" is not at all uncommon among birds. "Unfaithful" females benefit from the best of both lifestyles; they get mated partners to help raise the chicks and also a more varied genetic contribution to their offspring than they would receive from their mates alone.
In mammals, as the article points out, it's impossible (without bottles and formula, at least) to divide parenting duties equally between male and female. Gestation and breast-feeding help to account for the much lower frequency of monogamy among mammals.
Elaine Morgan's THE DESCENT OF WOMAN outlines the factors common to most species that practice pair-bonding: (1) Helpless infants who require intensive care in early life. (2) A den, nest, or other fixed location where the young are sheltered. (3) Approximate equality between male and female rather than overwhelming male dominance (which the BBC article also mentions).
An additional, rather grim purpose for pair-bonding arises from the tendency of males of many species to kill infants sired by other males. A female with a permanent mate has protection for her babies against marauding outsiders.
On Marion Zimmer Bradley's Darkover, the original colonists from the stranded ship in DARKOVER LANDFALL recognize the perils of settling a new world with a limited gene pool. Therefore, in the early generations of Darkovan society, women are encouraged to bear children by as many different men as possible rather than entering into exclusive marriages. Of course, they also receive an outside genetic contribution from the chieri.
Despite the benefits of genetic variation to the family and the species, faithful monogamy remains the ideal in our culture. Most of us probably believe the social, spiritual, and emotional motives for love and marriage outweigh the advantages of spreading our DNA abroad.
Margaret L. CarterCarter's Crypt