It's especially a pointless question for humans because, unlike any other species, we have learned to decouple sexuality and fertility. In prehistoric times, it was in our biological interest for a child's parents to be committed to one another to ensure the survival of the child to adulthood and thereby, the continuation of the genes. In fact, the extended family was even better (and was, historically, the most common form of childrearing) for exactly the same reason.
But that's fairly irrelevent to us now. Since we have largely divorced sex from fertility, monogamy now becomes simply one option among many. For some people, monogamy comes naturally.
Interesting point. But the term "monogamy" is linked not to sexual activity but child raising. For the non-breeders, to use a gender neutral term, it seems to make little difference to the society what form of sexual expression is chosen.
However, for those who chose to accept responsibility for children whether in the usual way or by adoption, a stable family commonly reinforced by sexual bonding is an important value for society to reinforce. Unfortunately both civil and religious mores are far behind the curve on this critical issue.
I would like to see "marriage" as permission for sex completely thrown out of both civil and religious laws. The state would create family unions to protect those who choose to form families for the purpose of raising children. Religions might want to restrict "marriage" to those couples with a family union license from the state. These unions would be structured to protect the family unity with a bias toward protecting the children in the event of a separation of the adults in the union.
Social units not involving children can be handled better via contractual arrangements, pre-nups, visitation rights, wills, etc. I doubt that religions would want to be involved in blessing such arrangements.