Why Are We Monogamous? And Are We Supposed to Be?
Are humans meant to be monogamous? That question has been debated for many years and while there isn't any one answer, there are many different theories. Scientists argue about how monogamy came to be - was it a romantic choice or did it develop as a means of survival?
There are two basic ways for males to ensure their genes are carried into future generations. One is to impregnate as many women as possible and the other is to impregnate one (or a few) and to protect the young to increase their survival rates. Based on these theories, monogamy didn't necessarily grow from "true love" but from the need to ensure protection of offspring.
Most mammals are not monogamous, and, in many human societies around the world, monogamy is not the norm. Roger Rubin, int the article, "Alternative Family Lifestyles Revisited, or Whatever Happened to Swingers, Group Marriages and Communes," says that out of 238 societies around the world, only 43 are monogamous. And throughout history there have been reports of wealthy and powerful men having hundreds, if not thousands, of concubines. This was accepted practice in past societies and the children from these unions were all considered legal and could inherit property.
The Roman empire, however, instituted laws against monogamy. Children born outside of a marriage were given a lower legal status and were not allowed to inherit property. And while divorce was not prohibited, it was not considered desirable. Some of these laws may have come about in order to make sure any single aristocratic family did not dominate the community and hold all the wealth in the community. Despite the reasons, the laws, traditions and customs have endured and today, in western society, monogamy is considered the norm.
Possible Reasons for Monogamy
A few recent studies have come up with two theories on why humans are monogamous.
In one study, researchers believe that monogamy evolved because other males, of the same species, would kill the offspring in order to mate with the mother. Mammals breast feed their young for a long period of time, leaving them out of the mating pool for extended periods of time. Males, competing for the females, would kill the offspring, making the female available for mating again. The study, completed at the University College London, looked at 230 different primate species and determined that, before social traits for monogamy developed, there was a high rate of outside males killing babies. Once the primates began pairing up with one specific mate, the rate of killing decreased. 
Another study, completed by Dieter Lukas of the University of Cambridge, also looked at how monogamy developed but he came to a different conclusion. He found that females began to spread out in order to find better sources of food. In an effort to keep other males from impregnating the females, males would stay close to the females. According to Tim Clutton-Brock, a zoology professor and co-author of the study, ates, "Males cannot successfully defend more than one female." 
Romance and Monogamy
Dr. Sue Johnson if the Alliant International University and the University of Ottawa, believes humans are naturally monogamous. She points out that monogamy is in cultures (human and animal) where time and effort is put into raising children. She also believes "we are wired for monogamy." According to Johnson, "A huge part of our brain is deisigned not just for social group interaction but for the intimate synchorony of emotional connection and bonding...Monogamous mammals like s have special cuddle hormones, like oxytocin or OT, the so-called molecule of monogamy. It turns off stress hormones, turns on reward centers and fills us with calm contentment and well-being. OT is released at orgasm and even when simply thinking of our partner...When scientists increase OT in little monogamous prairie voles, they cuddle more and mate less. When they block OT, they mate but don't cuddle. Our brains are wired for a certain kind of connection." 
Dr. Johnson explains that once we make an attachment bond, it is to one "irreplaceable" person and it is that person we seek closeness with. Our natural inclination toward monogamy, she says, is why, even when a married person strays, he or she often still fights to keep their relationship intact.
The reasons or evolution of monogamy may never be completely known or understood. Whether innate or learned, our civilization is, for the most part, monogamous. Despite the constant barrage of news stories about successful men and women having affairs, only about 25 percent of married people have an affair outside of their marriage. And the large majority of adults in the United States find extra-marital affairs to be morally wrong.
Alternative Family Lifestyles Revisited, or Whatever Happened to Swingers, Group Marriages and Communes," 2001, Sept., Roger Rubin, Journal of Family Issues
 "Monogamy: Are We - Can We Be - Monogamous?" 2010, March 12, Sue Johnson, Psychology Today
  "Scientists Find Roots of Monogamy Less than Romantic," 2013, July 30, Staff Writer, CBSNews
"What Makes Western Culture Unique?" Date Unknown, Kevin MacDonald, KevinMacD