The "Christian" definition of marriage

I find it quite amusing, especially nowadays (because gay marriage has somehow become such a big issue recently in many countries) how Christians define the "traditional" and "Christian" marriage as "between one man and one woman."

Where exactly is this definition found in the Bible? Sure, there are some passages that allude to marriage, but nowhere is polygamy prohibited. On the contrary, polygamy is not only allowed (and quite many of the greatest and most righteous men of God were polygamous) but there are laws governing such marriages, such as for example (Deuteronomy 21):

If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love.

There is, in fact, no passage in the Bible that forbids polygamy, or even recommends against it. So where exactly does this "between one man and one woman" come from?

A few Christians will argue that either those laws were for the ancient Israelites only, or that they do not hold anymore (because of Jesus or something.) This doesn't help much, though. Nowhere in the New Testament is this corrected or changed, and nowhere is it said that it's exclusive to Israelites and that gentiles have a different rule for this. What's worse, that argument would imply that God's universal, objective law about such an important thing as marriage does not apply to all of his followers equally or, what's worse, has changed.