Good question. They can't. U.S. Marriage law, solely by prohibiting polygamy, incest, and the forced marriage between a woman and her rapist, is not the law allegedly laid out in the OT by YHWH. "Traditional Blblical marriage" is between a man and his wife and his wife and his wife and so on. And for those who claim, "That doesn't count, it's OT law that Christ released us from with his death on the cross!" Anyone who has ever been divorced and remarried for any reason is not actually married according to God's law in the New Testament.
Second marriages aren't just adulterous; they're not recognized as marriages at all by Jewish law, which is what the whole "traditional marriage" argument is based on. Hence, they're adulterous relationships because a legal divorce can't nullify the original marriage in God's eyes, so any subsequent marriages, regardless of their legal status in the culture of their day, would not be recognized by God.
But I'm sure the movement to ban second and third marriages to preserve the "sanctity of marriage," is on its way as soon as they're done handling gay marriage. ;)