The logic behind polygamy is often not understood or if known than rejected and neglected. Islam is the only religion on the face of earth which actually gives those rights to women which are not even given to them by the modern day world. The provision of four marriages takes with it many reasons, its not just a man is given right to enjoy his relations with four women.
With the passage of time, the population of women is increasing day by day and the population of men is decreasing. And by the survey and prediction of the researchers, 100 plus years from now, there will be 50 women for one man. There are very few countries in which women are less than men. There are more women than men is many countries. So, one reason for the provision of four marriages is the reason of fewer men for more women. It is better to be wife then to be a commodity for men, who use women and then throw them like tissue papers.
Second reason is the mortality rate which is higher in the boys than girls. There are more baby boys deaths than baby girls, similarly more men die as compared to women. This means fewer men live for more women.
When a man marries and he does not get off springs from his first wife, he can marry and have children from his second wife. There is no sin in it. if the race stops at a point, the family will be over, so in order to keep the race and generations continuo, man can marry but not up to four wives.
If a man can afford more than one wife, both in money and in treatment, than he can marry more than once up till four. Many people raise questions on this point. but if they realize the logic behind this, they will start praising it. since the generation goes on by th male heirs, if they marry and have more children, they will be abel to have more muslims and their children in turn will produce more muslims. Hence muslim population increases. If you search a bit, you will see the biggest concern of non muslims is the child birth in muslim society. To control the ever growing muslim population, they have launched the child birth control program long back especially in the muslim countries.
So, polygamy is not a sin but it is a solution to the problems of the society. Men find women and women find men and in this way they keep themselves away from sins which otherwise they can fell into.