The most common verse in Quran which refers to the topic polygamy is Qur'an, Sura 4 Ayah 3 (Surat An-Nisa)
And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice].
http://quran.com/4/3
No matter what excuse Muslims give to defend polygamy but the fact is that in olden times polygamy acted like a very effective tool to convert more and more girls into Islam as Muslim had the freedom to marry up to 4 girls and produce more and more children.
In those days slavery was legal ( Arab slave trade http://en.wikipedia.org/wiki/Arab_slave_trade ) Girls captured from enemies were taken by Muslims many of them put into slavery. So girls were easily available and having more than one wife was something common with Muslims. If you look at various Muslim kings in India like Akbar, Jahangir etc.. they also had multiple wives.
From the very beginning it has been a tactic of Islam to spread rapidly and polygamy definitely served as a tool.
Till today so may Muslims in India have 5-10 children in spite of the country suffering from over population the government doesn't seem to show any sign to control it.
Polygamy is illegal in India for Hindus and other religious groups but it is legal for Muslims. It seems as if Indian laws do not apply for Muslims and laws are made on the basis of religion. Well laws of a country should not be made according to any religion whether Islam, Hinduism or Christianity. Laws of a country should be equal for every citizen regardless of his/her religion. Although there are movements to end polygamy, many Muslims seek to preserve the practice.
