Marriage is a union of man to a woman. When people marry for love, there is no dout that family will be happy. It is impossible to love several women at the same time. Loving somebody is almost the same to treat him or her failry. In my opinion, polygamy will be obstacle for it. Can you imagine a life children from different mothers? Do the friendship and respect exist among them? It may, but very rare, right? Children are born to be happy, not suffer from abuses of stepbrother and sisters. I am meantioning this, because nowadays victimis of sibling abuse have increased. If we legalise the polygamy, the situation will be worse.
The ability to marry people who are already married presents us with the chance to take partners that have already proven themselves in society as responsible parents and providers - that is, to reduce the risk in marriage. It also means that one doesn't have to settle for those "left over"; you can marry the person you want, not just the person that's left. Very few countries have exactly the same number of males and females; in the UAE, there are 2.2 males per female, but in Estonia, there are only 0.84 males per female1. This means that monogamy restricts options significantly.