Still, I'm left with six verses on the "Yes" side to deal with. Some of them I have already commented on briefly in the past, saying that they're really in the wrong column. The passages from Titus and 1Timothy that talk about a "bishop" being "husband of one wife" are making the point that the most spiritually mature people will avoid polygamy. (Many modern churches also interpret this to mean that divorced people should not be pastors.) The verse from Matthew is a cultural misunderstanding, as these "ten virgins" are not brides, but bridesmaids. They're waiting for him not to marry him, but to escort him back to where the wedding feast is being held.
That leaves me with three to explain on the "Yes" side, and at this point, I'll make a statement that may surprise some: I don't believe that polygamy is a sin. While the verses on the "No" side are applicable to polygamy, many of them are really talking more about the sanctity of marriage and the hurt that is caused by divorce. More pointedly, none of them come right out and say, "Thou shalt not have more than one wife." I think polygamy, like divorce, is an institution that the Bible allows, but never really gives approval for. In a perfect relationship, a man would find himself one and only one wife, and he would marry her and stay married to her until one of the two of them dies. However, we don't live in a perfect world, and men are slime. Boo.
Well, among those three remaining verses, I think there are two categories. The excerpt from 2Samuel is more of a statement of fact than an endorsement. David had multiple wives, some of which he had got from Saul, the previous king. I don't know for sure, but it may be possible that rather than David marrying the wives of Saul, this refers to him marrying the daughters of Saul, which he definitely did. The matter of polygamy still stands, though, and in this case, it's an important one. In Deuteronomy, God set up some rules for the future monarchy, and among those rules was the rule that the king shall not "multiply wives". While this may mean that he should not have an excessive amount of wives rather than an outright ban on polygamy, David had probably somewhere around a dozen wives, and of course his son Solomon was famous for having 700 wives and 300 concubines, so I think both of them--but certainly the latter--qualify as having excess here. (Actually, David and Solomon disobeyed a lot of those rules for kings, if you read through them; and it became their downfall, really.)
The remaining two verses come the closest to endorsing polygamy in the Bible, but I think they need to be looked at in full rather than in the part that are quoted on the polygamy page.
While these two verses allow for polygamy, the main point of both is a provocative one. God is saying to the men of Israel that they are only allowed to take another wife if they can keep themselves from playing favorites. Consider in particular the phrase "her duty of marriage, shall he not diminish." I could be wrong, but I think God is saying that if you have sex with your wife four times a week, that doesn't mean getting a second wife means you can sleep with each one twice a week; you need to keep your first wife's, ahem, "duty of marriage" at the same level. 'Nuff said.
"If he take him another wife; her food, her raiment, and her duty of marriage, shall he not diminish." -Exodus 21:10
"If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his." -Deuteronomy 21:15-17
This presents an interesting challenge to the would-be polygamist. Can you not only treat all your wives the same, but keep treating your first wife just as good as you did when she was your only wife? That's a tall order! I'm going to take a tangent into the Quran for a moment, and call your attention to this page. Although I have done it on the SAB forums a couple times, this will be my first time here in the blog: I'm going to defend the Quran in this matter. Note that the first verse is a conditional! A friend of mine, who is not a Muslim (or a Christian, but apparently is as much interested in religion as I am if not more) tells me that some Muslims, and in particular the laws of the country of Morocco, take this apparent contradiction to be an implicit ban on polygamy!
Statement: You may have a second wife if you can treat both your wives fairly and equally.
Statement: It is not possible for a man to treat two wives fairly and equally.
Conclusion: You may not have a second wife.
Getting back to the Bible, by similar logic it is concluded by many that while the Bible does not explicitly state that polygamy is a sin, it is implied by the Bible that anything other than a lifelong monogamous commitment between one man and one woman is less than God's ideal, and will lead to heartache.