The human race has been undergoing so many changes since its origin. He has come a long way from animal like forest living to the present day civil society living! In this process, he has moved away from the nature and the laws of nature too. However, being a part of the Mother Nature, it is not easy to stay away completely from its influence. Hence, there has always been an ongoing conflict at a personal and social levels about the rules of the ‘civilised’ society and that of the nature!
In nature, living beings have two basic stages. Namely, developmental stage-where they grow and develop into a fully matured being and reproductive stage-where they reproduce for the purpose of continuation of the species. Continuity of the species depends on how fit they are to survive against their enemies or unfavourable conditions. So, they have to evolve into a better version of the species in each generation (just like updating the software in each version). Thus, producing a better generation is an important natural instinct of all living beings.
If we look back into how we were and also into how animals live by observing the nature’s law, there is a conflict between human’s natural instincts and civil regulations or cultural inhibitions. If you observe animals then you will see that the female tries to attract many males and males will prove themselves stronger in order to mate with the female. So the female will choose the strongest male, from all those competing ones, to reproduce. This is because, she wants to produce a stronger progeny and thus making the species to be strong enough to survive. This is nothing but a natural instinct for the survival of species.
This attitude is not so obvious in humans due to social inhibitions, ethical thinking etc. However, there is an indirect inclination towards choosing a strong male either physically or socially (who can sustain financially and take care of the family). When any of these factors are not supported and the female suffers from insecurity, then it results in emotional disturbances and a tendency to look for another male to support her!
Now, if you see how the nature has designed male and female then we can understand why males tend to be more polygamic than females! Females have limited number (400-500) of ova (eggs) for the lifetime. But males have millions of sperms in each ejaculation! A female is designed to release only one ovum every month. Whereas millions of sperms battle to fertilise that single ovum and only one succeeds sometimes (not always)! This again supports the reason why many males compete to win a female. Then, once conceived, the female won’t be able to reproduce for the next 10-12 months. So, the male who still can reproduce and has got millions of sperms, tries to have multiple female partners so that the process of reproduction continues without a break! After all the very basic instinct of natural phenomenon is to survive and sustain, and the basic process to achieve this is to reproduce and have stronger generations in the act of evolution.