If you look at what each sex needs to do to have children, monogamy for men doesn't make much sense. Fewer than three percent of mammals are monogamous, giving them a much better chance to pass on their genes. Humans, on the other hand, are an unusually faithful species. "Probably a majority of people do engage in long-term, committed monogamous – and sexually monogamous –relationships," says David Buss a professor of psychology at the University of Texas and author of three books on sex and evolution. "But a pretty hefty minority don't." Although there are evolutionary reasons why men would want to sleep around, there are also uniquely human characteristics that may favor more committed relationships.
In most mammals, the minimal investment males have to put into having a child is one bout of sex. So, basic evolutionary logic tells us that they should just sleep with as many partners as possible. This may be why most males have evolved to crave sexual variety, whereas females are more likely to seek out a partner who can provide for her and her children. These sex-specific preferences apply to most mammals – us included – but we also have some additional traits that may encourage a more monogamous life.
Compared to other animals, it's hard to tell when human females are ovulating. So, in order to have a good chance of procreating, a guy has to have sex with a woman over an extended period of time. Also, human babies are incredibly helpless and, over our evolutionary history, they've had a better chance of surviving if both parents raise them. A man who sticks around through mating and beyond, therefore, is more likely to pass on his fatherly genes.
Of course, only talking about lifelong monogamy and sleeping around is oversimplifying modern human sexual behavior. "We live in a very weird modern world, where short-term mating is possible in ways it never would have been ancestrally," says Buss. With access to thousands or millions of potential mates some people have affairs, some are serial monogamists, and others are in committed polyamorous relationships. Each of these, from an evolutionary point of view, has its pros and cons for both sexes.
In general, it may seem like evolutionary theory is encouraging men to cheat or sleep around but that conclusion misses the point of evolutionary science. "A deeper understanding of our evolved psychology is a good thing because it does help us put things in perspective," says Buss. Evolution can give us insight into why we are inclined to feel certain urges or emotional reactions and we can use that insight to make more informed decisions. But in the end, evolution doesn't force our hand. Our actions are still our own.