So when it comes to polygamy, we should know that it’s wrong, and we all do... this is precisely why we’re marvelling at the fact that Solomon had 700 wives! And the Bible does explicitly tell us in Deuteronomy...
And he [the king] shall not acquire many wives for himself, lest his heart turn away... (Deuteronomy 17:17)
And of Solomon, who had 700 wives and 300 mistresses, it later said “And his wives turned away his heart.” (1 Kings 11:3). We’re supposed to make the connection here, if we hadn’t figured it out for ourselves already, that Solomon had done wrong. And while the Bible never explicitly condemns Jacob or King David for their polygamy, we’re supposed to read those stories knowing that they were doing wrong; and when you do, you pick up on how their sin affected their lives, and we see characters suffering over jealousies and related issues. We ought not to forget that for as many examples as there are of polygamy, monogamy is still the norm throughout every stage of Biblical history, such as with Moses, or with Abraham (despite the incident with Hagar which is explicitly acknowledged as error), or with Noah.
The Bible does explicitly tell us that the intent for marriage is to be between one man and one woman. It tells us this from the beginning in Genesis 2... “Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.” (Genesis 2:24). And whenever the Bible speaks of marriage in a prescriptive sense it assumes one wife, such as in Malachi 2, and in New Testament passages like 1 Corinthians 7 or Ephesians 5. In fact, the point these passages are making is to parallel marriage with our relationship to God, and what it’s saying about our relationship to God is that it is exclusive. The Church is “one body”, and “betrothed to one husband”.
Nevertheless, whereas many Christians, I think, try to explain all instances of polygamy in the Bible as “they were doing wrong”, I’d say we need to be aware of at least one situation where polygamy is allowed for. There is the case of levirate marriage in the Bible. This is a practice which is not unique to the Bible, but has been known in many cultures around the world. This is the case where, if a man dies and he has no children to carry on the family’s heritage, that man’s brother is obliged to marry his widow, and the widow is obliged to marry the brother. And it seems that this obligation exists even if that brother is already married, leading to a case where polygamy is not only permitted, but where there is an obligation to enter that arrangement. It doesn’t seem to be an enforced commandment, though, since the Bible speaks of the right for the brother to refuse, and the only consequence of this is that the brother should be ashamed of himself (Deuteronomy 25:5-10). But given the allowance for levirate marriage, when we come to read about a man named Elkanah in 1 Samuel who had two wives, perhaps we ought to be at least a little uncertain as to how we should judge him. It may be, though it doesn’t say, that he necessarily had two wives through a case of levirate marriage? We don’t know. And where the Law says “If a man has two wives, the one loved and the other unloved...” This passage goes on to speak about inheritance, and not favouring the firstborn of the loved wife. But to me it makes the most sense that this is said in the context of levirate marriage, where inheritance is the primary issue, and where a man is surely going to love one wife, and possibly not be terribly in love with the woman he was obliged to marry through the levirate custom!
So I think that in so many cases where the Bible speaks of men married to two women, levirate marriage is probably the unstated, implicit reason for it. And there's reason, from the Bible, to believe that levirate marriage was probably the only valid exception to a monogamous marriage. Look at the following passage where the religious experts come to Jesus, trying to catch Him out teaching something perverse... They say:
"Teacher, Moses wrote for us that if a man's brother dies, having a wife but no children, the man must take the widow and raise up offspring for his brother. Now there were seven brothers. The first took a wife, and died without children. And the second and the third took her, and likewise all seven left no children and died. Afterward the woman also died. In the resurrection, therefore, [listen to what they say here] whose wife will the woman be? For the seven had her as wife." (Luke 20:28-33)
"Surely the woman can't have more than one husband!" is the argument, and this argument is based on the Law of Moses! Jesus answers that nobody is married in the resurrection. But we see that even for the people under Moses, monogamy was the standard, with levirate marriage being probably the only valid exception.
Levirate marriage is still no excuse to justify polygamy today. It applied to Israel in particular because God had a purpose in keeping the separate tribes of Israel somewhat separate, and ensuring that inheritance was kept within each tribe. The reason for this, as far as I can tell, though there is probably a better answer out there, is that part of the reason we know Jesus Christ was the Saviour to come is because He fits the criteria according to the prophecies... that He would be from the tribe of Judah, and born in Bethlehem. So by the time of Christ there still had to be a tribe of Judah, and Bethlehem had to still be identified with the tribe of Judah.
Finally, I want to look at Jesus’ “Sermon on the Mount”. Here he says, “everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.” (Matthew 5:28). The format of Jesus’ arguments in this part of the Sermon is like this... “If you think this or that is a significant sin, be aware that every sin starts in our thoughts, or intent.” How would we get to the stage of actually committing adultery, physically, if we prevented it getting further than thoughts becoming intent? As regards polygamy, surely the initial intent is adultery, and the only difference between adultery and polygamy might be that our (first) wife consents to the relationship with the “other woman”. But I would say that this is no less adultery, especially from God’s perspective. We know this because Jesus said in another place, “whoever divorces his wife and marries another, commits adultery.” My point here being that even if the wife is out of the picture, and is quite happy for their ex husband to have “moved on”, this is still adultery in God’s eyes. If having another wife is a sin after you’re divorced, how much more is it a sin while you’re still together!? Even if your wife consents, it only means that you’re both committing the same sin together.
You shall not covet your neighbor's wife. (Exodus 20:17)
