No where in the Bible is it said that you may have only one wife. To be honest is is not a religious thing. It is more of a social thing that happened.
Actually, the bible specifically allows multiple wives and many of the central characters had multiple wives.
For example
In Exodus 21:10, a man can marry an infinite amount of women without any limits to how many he can marry.
In 2 Samuel 5:13; 1 Chronicles 3:1-9, 14:3, King David had six wives and numerous concubines.
In 1 Kings 11:3, King Solomon had 700 wives and 300 concubines.
In 2 Chronicles 11:21, King Solomon's son Rehoboam had 18 wives and 60 concubines.
In Deuteronomy 21:15 "If a man has two wives, and he loves one but not the other, and both bear him sons...."
http://www.biblicalpolygamy.com/ for more