While monogamy is adhered to by many, some religious groups encourage polygamy. Proponents of polygamy often point to the fact that numerous men mentioned in the Bible had more than one wife.

Jacob had two wives and two concubines. Others with multiple wives include Moses, David and Solomon.
However, when we read the New Testament we see monogamy was chiefly practiced by Christian believers.
So, has God changed the rules or are most ignoring a Biblical prerogative?
Carefully Reading the Holy Bible
A point one must consider when reading the Holy Bible is that the reader needs to discern the difference between what the Word of God records and what it actually sanctions. The Bible certainly contains history, but that does not mean every event and action was God honoring or approved by God.
When we carefully consider all of those in the Holy Bible with multiple wives, we find that nowhere does God approve polygamy. In fact, most that did have more than one wife were subjected to domestic or spiritual problems as a result of their multiple wives.
Reading consecutively through the Old Testament you will see polygamy is recorded less as one progresses closer to the advent of Christ. Mention of it in the New Testament is rare, and there is no indication that any of the Apostles or outstanding disciples had multiple wives.
Polygamy Came After the Fall of Man
Context is another factor to consider. Polygamy seems to be an invention of man. The first Biblical record is in Genesis 4:19: "And Lamech took unto him two wives: the name of the one was Adah, and the name of the other Zillah." However, instead of us looking a Lamech or the other aforementioned men we really need to look at what God did.
Even those vaguely familiar with the Holy Bible will know that God created the first man and woman. It is important to note that when God did create a helpmeet for man, He only made one. He also made that helpmeet as what might be described as the opposite gender.
Nonetheless, He only made one, and a careful reading of the balance of the lives of our first parents will show that neither Adam or Eve had additional spouses, even after sin entered into the world.
It strongly appears that polygamy was an invention of man in part to satisfy sensual and economic lusts. Prior to the account of the flood, we read in Genesis 6:5, "And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually."
Man, of course, continued to be sinful after the flood with sexual gratification outside of or against marriage being one of the main evils.
However, Christ Himself, in dealing with questions regarding divorce (see Matthew 19:3-12), focused upon the Genesis 2 where, as previously stated, we see God created only one man and only one woman to be together for life. Though He was addressing divorce, the very fact that He used the Genesis 2 account points out that monogamy, and not polygamy, was the plan sanctioned by God. Christ could have easily mentioned Jacob as a pattern, but He did not, leaving one to conclude God's choice is monogamy.