From an evolutionary sense, no monogamy is not "natural" for humans. Though, I would be hesitant to use the word natural, as it implies a moral value. Humans show sexual dimorphism (differences between genders). Specifically, the size of humans points towards our polygamists routes. Males, obviously, are much bigger than females for many reasons: protecting and guarding mates, fighting between individuals, securing resources, etc. Other evidence pointing towards the swinging sexuality of humans is that fact that nearly 15% of the children born are born to the incorrect genetic fathers. In other words, about 15% of children are raised to father's who, unknowingly, are raising offspring that are not their genetic kin (the research is out there, trust me). Why is this? Humans, and every other organism out there, is selfish in an evolutionary sense. Our genes, unconsciously and unknowingly, shift us to do what is right for our genes. That could be cheating on a spouse as to secure better genetic material, or to spread one's genetic material through unconscious drives. These drives are fitness enhancing (look up the definition of fitness in a biological sense). Of course there are individuals that fall outside of the norm, but in general, individuals take advantage of opportunities when they are advantageous to their genes. So are humans strictly monogamous? No. Does this make it right to cheat on a spouse? Absolutely not. Biology does not justify what is right or wrong; those are values that our society decides.