The Old Testament:
May your fountain be blessed, and may you rejoice in the wife of your youth. --Proverbs 5:18
Has not [the LORD] made them one? In flesh and spirit they are his. And why one? Because he was seeking godly offspring. So guard yourself in your spirit, and do not break faith with the wife of your youth. -- Malachi 2:15
In the New Testament, one-man-one-woman marriage is distinctly Christian. Paul made marriage to one woman a necessity for role models in a church:
A deacon must be faithful to his wife and must manage his children and his household well. --Timothy 3:12
Nobody is making the essential legal argument...
Reynolds v. United States (1878) defined marriage as one man and one woman.