I believe polygamy is a sin. 4enlightment provides some good scriptural quotes proofs for this above, particularly Matthew 6:24.
God can forgive you of your sins, remember that. Consider though why polygamy is sinful. Remember God did not make sin, sin is merely the acting contrary to God. God does not list sins arbitrarily, but He tells us what they are because He is aware how different sins cause people suffering.
For example, all the human heroes of the Bible are guilty of sin, this is fact. The Bible tells you the Truth, and the Truth is not always pleasant, but it has to be told. David is a good example, especially when he sinfully woos Uriah's wife thus forcing the Prophet Nathan to pronounce a prophecy from God saying that the sword will not depart from his house, as well as many other things that happen after David does this that threaten to ruin his kingdom and cause David great sorrow. Solomon is also an excellent example of how polygamy is unwise as his many wives caused a return of idolatry in Israel, thus angering God and setting in motion the division of Israel and Judah after Solomon's death, and thus the eventual conquest by Assyria and Babylon. Jacob (whom God renamed Israel) married both Rachel and Leah (though in fairness this is due to a trick of their father Laban) also displays how polygamy hurt Rachel and made her envy Leah her sister and put enmity between the two, and also set in motion the future envy between Joseph and his brothers Abraham sleeping with Hagar show how this hurt Sarah (even though she herself suggested the act) and showed the great harm it did to Hagar and Ishmael. So the examples go on and on in the Bible of the hurts of polygamy. Even in today's World you can see this.
Let us also consider the Biblical origins of marriage. In the beginning Adam and Eve were made one for the other and were happy. God intended for One Man to be with One Woman. However as we know the Bible story well, we know that Man chooses time and time again to forsake these things (thus sin.) The same even is true for their immediate progeny, Seth and Cain.
Who is that first polygamist? The Bible actually have an answer. If you look at Genesis Chapter 4, after Cain slays Abel he is cursed to wander the Earth and goes East of Eden to the Land of Nod. The first polygamist comes from the line of Cain. The first polygamist is La'-mech son of Me-thu'-sa-el, son of Me-hu'-ja-el, son of I'rad, son of E'-noch, son of Cain. (Not to be confused with La'-mech father of Noah of Seth's line.)
La'-mech, of the House of Cain, is also the second murderer named in the Bible. Since his story is short here is his story as told in the King James Version of the Bible:
Genesis 4:19 La'-mech took unto him two wives: the name of the one was A'-dah, and the name of the other Zil'-lah.
20: And A'-Dah bare Ja'-bal: he was the father of such as dwell in tents, and of such as have cattle.
21: And his brother's name was Ju'-bal: he was the father of all such as handle the harp and organ.
22: And Zil'-lah, she also bare Tu'-bal-cain, an instructor of every artificer in brass and iron: and the sister of Tu'-bal-cain was Na'-a-mah.
23: And La'-mech said unto his wives, A'-dah and Zil'-lah, Hear my voice; ye wives of La'-mech, hearken unto my speech: for I have slain a man to my wounding, and a young man to my hurt.
24: If Cain shall be avenged sevenfold, truly La'-mech seventy and sevenfold.
(Just some food for thought. Heh, complicated things like this remind me why I am glad to be single.)