The Bible does not teach that marriage is a one man/one woman deal. There is far more polygamy than monogamy in the Bible, and it is not condemned. . . . If we want people to take us seriously, we need to handle Scripture seriously, and claiming that the Bible teaches "one man/one woman" is simply not accurate. The Bible does clearly teach that homosexuality is a sin, but how can we expect anyone to believe us if we lump it in with something where we are being (at best) sloppy or, (at worst) insidiously misleading about what the Bible actually says?(By the way, Ruth stressed that she wasn't in favor of having more wives in her house.)
Now this is—or ought to be—a common thing to consider. (In fact, I once had this conversation with an Orthodox Jewish politician when we were discussing same-sex marriage.) Why do we categorically call polygamy wrong when great heroes of the Bible were polygamists?
The answer begins with the basis Jesus and the apostles commonly used for their discussions of marriage: Genesis 2:18-25. In particular, note how Jesus handles the issue of divorce:
Then some Pharisees came to him in order to test him. They asked, "Is it lawful to divorce a wife for any cause?"What's so interesting about this is that when confronted by the Pharisees with Moses' God-breathed legislation on divorce, Jesus brushes it off by pointing to the way it was "in the beginning." In the beginning, a man and a woman were united by God as one flesh for life. The pattern of divorce that came later was a concession because of people's sin. But now Jesus says that that concession is invalid. Now God demands that we go back to the way it was in the beginning.
He answered, "Have you not read from the beginning the Creator made them male and female, and said, 'For this reason a man will leave his father and mother and will be united with his wife, and the two will become one flesh'? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate."
They said to him, "Why then did Moses command us to give a certificate of dismissal and to divorce her?"
Jesus said to them, "Moses permitted you to divorce your wives because of your hard hearts, but from the beginning it was not this way. Now I say to you that whoever divorces his wife, except for immorality, and marries another commits adultery" [Matt. 19:3-9].
It seems to me that Jesus' approach to divorce applies neatly to polygamy also. Polygamy is every bit as much a violation of the "one flesh" principle of Genesis 2 as divorce is. "In the beginning" Adam was lonely and needed a companion. God took his rib and fashioned the woman from it. Adam recognized her as his own flesh and bone and became reunited as one flesh with her again. She was God's sole and sufficient answer to the problem of his loneliness. Adam needed someone other than himself to become one with; once he became one again, he did not need to become more than one, which a second wife would imply. In fact, though Jesus does not state this outright, it seems that it is because of the logical impossibility of becoming exactly one with multiple women that Jesus cites "immorality" as the sole ground for divorce. Sexual infidelity is literally the only thing that can compromise the oneness that God made when the couple was married. Becoming one with a second, if perpetuated, replaces the oneness with the first.
However, it seems that, as with divorce, God ignored polygamy for a long time as a concession to humans' sin. The first time polygamy appears is with the abominable Lamech, the seventh generation of the line of Cain and the representation of the "perfect" outcome of that evil lineage.
Now contrary to Ruth's assertion, there is actually very little polygamy in the Bible. The only way there could be is if there were throngs of unattached men in ancient Israel or if men succumbed to disease far more easily than women did, and we have no evidence of either. Rather, polygamy was reserved for the exceedingly wealthy who could actually support all the wives and children, and later on only kings. In the Greco-Roman world in which Jews lived, polygamy was unheard of, though divorce, mistresses, prostitution, homosexual affairs, and (for the Greeks) pedophilia outside of marriage were common.
But in all of the few times that polygamy does appear in the Bible, even among those most honored by God, we see the deleterious results that one expects from sinful behavior. Abraham's employment of Sarah's slave Hagar as his concubine led to the rivalry between Ishmael and Isaac that ominously foreshadowed the conflict between their descendants (Arabs and Jews respectively) today. Jacob's two wives and two concubines fostered intense and almost deadly sibling rivalry. Elkanah's second, fertile wife, Peninnah, caused immense distress for Hannah, though the unloved Peninnah was surely miserable as well. David's multiple wives led not only to the power struggle surrounding the succession to his throne, but also to the open rebellion and civil war against him stemming from his son Absalom's revenge for David's complacency over the rape of Absalom's sister Tamar by their half-brother Amnon. Worst of all, Solomon's acquisition of a harem directly led to his apostasy to worshiping other gods, because of which Israel was split in half, and neither part ever fully recovered (in an Old Covenant sense).
These are consequences of the sin of polygamy that God ignored for a long time. This isn't the only example of God not making a big deal out of part of his will as he progressively works with his people. For example, God told Israel through Moses not to sacrifice to him anywhere in the promised land except for the place where he dwelled in his tabernacle. But sacrifice to Yahweh at the "high places" continued for centuries, and God blessed it (e.g., 1 Sam. 7:2-13; 1 Kings 3:1-15; 18:16-46). But eventually God enforced his previous dictum. Likewise, with respect to polygamy, by the time Christ came God's word was, "Enough." It was time to go back and do things as he had established in the beginning.
So despite examples of polygamy in Scripture, the Bible as a whole really does teach that one-man/one-woman marriage is what a marriage really is.