Because it is. By our very nature we are not monogomous. Men were biologically designed to have several female partners. Hence why we can orgasm like every ten minutes whereas it takes nine months for one woman to have a child.[Trigger Warning: This post contains Theory of Biological Evolution]
Let us first examine the closest living genetic relative of the Homo sapiens, the Chimpanzee, because I told us to.
Chimpanzee males are much larger and stronger than Chimpanzee females. Chimpanzee males develop crests as a sign of dominance. Chimpanzee females express visible signs of ovulation. The Chimpanzee males fight over ovulating females and the stronger one gets the female. This results in many "polygamous" situations.
Now, let us examine the homo sapiens:
Homo sapiens males are not, comparatively, much larger or stronger than Homo sapiens females. They do not develop crests as a sign of dominance. The females do not express visible signs of ovulation; human males, therefore cannot fight over ovulating females.
Why did homo sapiens evolve this way, and what does it say about their 'natural' sexual practices?[Trigger warning: Investigation in progress among biological anthropologists]
Recent discoveries have overturned the idea that hominids evolved to walk on two legs in order to walk and see across distances over savannah land. One of the oldest bipedal hominids walked on two legs in the forest. Because you can carry more food that way.
Who is carrying food? The males. Why? To feed the female and her children.
Chimpanzee males ensure that they have the largest possibility of being the father of as many children as possible by mating with as many ovulating females as possible. But a hominid male invests a lot of time an energy in feeding his children in order to insure their survival. What stops a stronger male from simply impregnating his female and making the providing male a cuckold while he's not around?
Because hominid women don't show visible signs of ovulation, other males do not know when the female is fertile. So they don't know if mating with her will produce offspring from that mating or not. But this still leaves a problem: What makes the providing male more likely to impregnate the female on random chance than another male who mates with her?
Hormonal Pair Bonding, that's what.
Studies in homo sapiens have shown that pair-bonded partners have linked hormone cycles. Male testosterone peaks when female fertility peaks. This ensures that the pair-bonded, providing male has the greatest chance of impregnating his female and providing for his own children.
So, in short, the so-called "default" mating behavior of the hominid is serial monogamy
with occasional straying. Interesting; that's just what you see today.