The Common Good offers these suggestions for a proper defense of Biblical Marriage (in addition to restricting marriage to one man and one woman):
Marriage shall not impede a man's right to take concubines in addition to his wife or wives. (II Sam 5:13; I Kings 11:3; II Chron 11:21)
A marriage shall be considered valid only if the wife is a virgin. If the wife is not a virgin, she shall be executed. (Deut 22:13-21)
Marriage of a believer and a non-believer shall be forbidden. (Gen 24:3; Num 25:1-9; Ezra 9:12; Neh 10:30)
Since marriage is for life, neither this Constitution nor the constitution of any State, nor any state or federal law, shall be construed to permit divorce. (Deut 22:19; Mark 10:9)
There are more, but you'll have to follow the link to read them - it wouldn't be right to quote them all, even if they are just faithful reporting of what appears in the Bible. For some reason, though, I don't think that we'll be seeing many people arguing for a constitutional amendment to define marriage in ways like the above... I wonder why? After all, each of the above remains legal - so they must be at least as much of a threat to marriage as is gay marriage, which isn't legal yet. Could it be that opponents of gay marriage are simply picking and choosing which aspects of the Bible they are going to take seriously?