"The Bible never condemns polygamy or having concubines. Is there a moral law against them?"
Yes there is a moral law against them because the Bible actually does condemn them. Jesus Himself said, “From the beginning it was not so…” He commands that marriage be between one man and one woman. He notes also that the relative lax standards for divorce in the old covenant were the result of our “hardness of hearts.” (See Mark 10 for this discussion.)
There is irony here. Most of the time we squirm over the harshness of the Old Testament and find the New kinder and gentler. Here Jesus narrows radically those circumstances where divorce might be permitted, and in the process rules out polygamy and concubinage. Which raises this question- what does Jesus mean by “because of the hardness of your hearts?” Why were these things seemingly permitted in the Old Covenant?
For all its radical calls toward complete holiness in our lives, the Bible also shows remarkable wisdom and grace in how it deals with our corporate sins. God, in the Old Covenant, though He established the pattern of one man and one woman in the Garden, and though it was the notorious Lamech who introduced polygamy, began to remake a world twisted by sin through regulating certain practices rather than ruling them out altogether. He established rules for how concubines and their children were to be treated. He established rules for not only the care of slaves, but for their eventual freedom. Even in the New Testament we see how delicately Paul deals with Onesimus, the runaway slave. He does not thunder from on high that all slavery must end, but does push toward that ideal in a more natural, organic way.
When we come to understand God’s patience with us we may be tempted to presume upon His grace. Or, we may find ourselves having to answer for what He permitted, as if He were the one that created slavery and polygamy, or the circumstances that led to them. Unbelievers delight to throw these cultural differences in our face, as if they prove something wrong with His character rather than exposing what it wrong with ours.
One thing should be clear from all this. Jesus was abundantly clear that God made marriage to be between one man and one woman. That does indeed rule out polygamy and concubinage. It also rules out that oxymoronic modern notion of “Gay marriage.” There is no such thing, and never will be. “Gay marriage” is not merely wrong, but is utterly nonsensical. We need not denounce “Gay marriage” any more than we need to denounce square circles, or noisy silences. Remember this nugget of wisdom- if we call a tail a leg, how many legs does a dog have? Four. You can call a tail a leg all day long but it doesn’t make it so. Dogs have four legs. Marriages have one man and one woman. Jesus said so Himself.
We would do well to remember this as well. Whenever something in the Bible embarrasses us, we should be ashamed. Not of the Bible, but of ourselves. God is true though every man be a liar.