Some conservatives and religious types understand the construct of marriage to serve certain social purposes as well--primarily procreation and the raising of a family. I'm not advocating that perspective (history shows this isn't always the case), just merely pointing out the conservative argument isn't always "the Bible says so[1]"--it is also an argument that society functions at its best within the "traditional" concept of marriage.

[1] It is worth noting that while there are passages in the Hebrew/Christian bible that advocated "one man, one woman" marriages, there are plenty of examples of "Biblical heroes" who had multiple wives (David and Solomon come to mind).