In nature, whenever male parental care is required (e.g. birds), the male wants to make sure that his female does not have sex with other males because he does not want to be a cuckold and raise another male's young instead of his own. It is the same with humans, guys do not want to be cuckolds. It has not always been this way. In fact, when we first evolved as a species, we were living in social groups of multiple males and females because of safety in numbers. Under such a system, males have to cooperate with each other to fight off predators. Therefore they have to be bonded just like soldiers. If they fight among themselves for females, then when danger comes, those who have been bullied will not help out others that are attacked. Females too want to have sex with many males because it is pleasurable and because that way her babies would not be harmed by males who never had sex with her and knew that they could not be the fathers. Back then, hunters worked as a group and they share the kill, so children could be raised communally. It does not matter who the father is, they all helped out raising children born within the group, and these children can grow up and become part of group doing the hunting or gathering. Of course females in such groups usually leave to join other groups to avoid excessive inbreeding. Males can stay because they help hunt.
Things changed when agriculture was invented. People no longer live a nomadic lifestyle and they no longer need other males to defend each other against dangerous animals. Each man can now settle down on a plot of land and grow enough to raise his own family. By disallowing his woman or women from having sex with other men, males in agricultural socities can make sure they have descendants. OTOH, humans who are still hunter gatherers continue to be promiscuous. Native Americans for example, allow women to have sex with different men when Europeans first arrived. As long as no incest was involved, native American women could have sex freely with anybody..
Things changed again in the 60s and 70s, when birth control pills and abortions were commonly available, and people no longer have to worry about raising unwanted children. The sexual revolution allowed both men and women to have sex without worrying about pregnancy. Of course, males still do not want to be cuckolds. That has not changed since laws now make men support children that they fathered. Child support payments can be pretty hefty. Nevertheless thousands of years of enforced monogamy have also reduced the willingness of females to have sex because those females who like sex with more than one men were often killed as adultery in many societies were viewed as crimes punishable by death. Even the Bible has many examples of adulterous women being stoned to death. Nowadays, some Islamic countries still kill woman for committing adultery. In a modern society such as Brazil, a man can often get away with murder if he kills his wife because she allegedly had sex with other men. It is called honor killing. As you can see, because of natural selection, harsh punishment of adultery and the fear of cuckoldry are the reasons why monogamy have been the norm traditionally in most societies around the world that no longer practice hunter/gathering lifestyles. Even a queen could be executed for alleged adultery in England a few hundred years ago, and Princes Di was supposedly a virgin when she married Prince Charles. The reason for monogamy is simply natural selection. Even a bird who sees his female have sex with another male may abandon her. Animals do not want to be cuckolds either.