Polygamy is allowed in islam there are hundreds of reasons, many of them are already listed by friends in this thread means,

1. Biological Reasons
2. Psycological reasons
3. Moral Reasons
4. ....51%..... In Pakistan female percentage is 51, so if polygamy is not allowed means that 2 percent of females can never marry in their life and have to spend life un-married. Furthermore Islam is for all times and it has already predicted that with the passage of time there will be more females as compared to male(1 male to many female proportion). So dont argue in ignorance.

Regards,