Monogamy is quite recent when it comes to human history. Indeed, although it appears naturally in some animals, in mankind it emerges as a social norm from the Neolithic. Monogamy was practiced by humans prior to this period but it was patchily adopted among hunter-gatherers. Form this time people started to cultivate crops, they were more inclined to choose monogamy and worship cults with women deities. Monogamy is design to “protect” a woman. A woman used to pass from being under the authority of her father or brother to that of her husband. When a woman was “given” to a man, her father was originally making an alliance with his son-in-law.
Monogamy is a contract between two people to fulfill different needs. The woman looks for protection and shelter whereas a man looks for the certitude that the baby that she carries is his. On top of this a man may look for free labor because most women in agricultural societies work much more than their husband. Monogamy is also recent in history because previously there used to be fewer men than women. Baby girls are slightly more numerous at birth than baby boys, this is true throughout human history (except nowadays in some Muslim countries as well as India and China, a lot of female babies are killed at birth which increase the proportion of men in the society). Also, warfare explained the rareness of men. This explains why even during the Neolithic, men continued to be polygamic to compensate for the unbalanced sex ratio.
But today, why are we continuing a worldwide pattern of monogamy? It’s probably because we are influenced by monogamous cults which shaped our institutions. However, there are other ways for a woman to find protection. Likewise, a man doesn’t need to exploit another woman to survive. He can also take a genetic test to make sure he is the father of a child. I would say that there are two possible reasons why society promotes monogamy. The first is capitalism. Indeed, monogamy promotes the family institution in which the wife and husband need to work to provide for their love ones. A family enables the possibly of accumulating wealth namely by the division of tasks between its members. A family has needs which are the basis of consumption. Without consumption, there is no need to produce goods and services which would stop capitalism otherwise. You may say, yes but it is the same for bachelors or gay couples who have needs.
Yes, indeed, but capitalism is based on growth, and demographic increase. Without demographic increase, the capitalist engine breaks down. You may wonder, would capitalism work in a polygamous or sexually free society? Yes, it would work for a generation only (that is, basically our current generation), but usually, for anthropologic reason a woman would not dare to have a child with a man if she knows she cannot have a guarantee in exchange (which is security through fidelity or wealth). The issue is that if the whole society became hedonist and polyamorist, women may not want have children even though they wish this because they wouldn’t get any guarantee. After one generation, the economy would collapse (using a capitalist approach) but it could be an opportunity for the human race since negative growth might be a solution of our economic problems.
The second reason why people are still monogamous is spirituality. People may not agree on that topic though. When one has sexual intercourse without love, only on the physical side, one may lose some subtle energies. Some may call it purity. The result is we are deprived of a vital energy which prevents us from achieving elevated goals. When we give of too much of our sexual energy, we can no longer sublimate it. The sublimation implies a transformation of sexual energy into elevated thoughts and actions. People are in one or other of these situations, the second is preferable because it is chosen personally and not conditioned by the society.