Just 3% of animals are naturally monogamous. Humans are not one of them. If infidelity is essentially having multiple partners, men cheat because they are built to.
Monogamy is the martial norm in much of the world. Yet, it is at odds with human nature. Anthropological research shows monogamy became established as the norm because large scale polygyny has disastrous social consequences. While our societies have developed around a monogamous ideal to limit discord, human nature has not changed. This dichotomy between how we live and how we are hardwired is problematic. Societies have engineered environmental factors - such as our legal system and the nuclear family idea - to uphold monogamous values and lifestyles. While these norms discourage cheating, they do not impact human biology. As CNN reporter Meghan Laslocky writes, "With people living longer than ever before, a greater tolerance toward the human impulse to experience sexual variety is needed. Whether a person succeeds at being sexually monogamous depends as much on biology as the environment."
It is reductive and misleading to claim that monogamy is just a construct. It is far more than that. Societies have chosen to organise themselves around this idea because of the dangerous side effects polygyny can have. As a report from the University of British Columbia found in 2012, "the intra-sexual competition that occurs causes greater levels of crime, violence, poverty and gender inequality than in societies that institutionalize and practice monogamous marriage." Societies without monogamy are therefore dangerous, unequal and poor. This is not the only way in which the positives of monogamy outweigh drawbacks. As an organising principle, it has many proven benefits across raising children, financial stability and mental health. Given how societies develop, there is very little about them that exists in nature. This is hardly surprising. More revealing is that scientific studies rarely point to the monogamy debate as the reason that people cheat. The case for marital norms driving infidelity is simply not strong enough.
[P1] Monogamy is unnatural for humans [P2] Having multiple partners is is natural for humans [P3] Cheating is natural for humans
Rejecting the premises
[Rejecting P3] Having multiple partners can take many forms other than cheating.