The Narrow has a long, post, with references, which argues from a non-biblical point of view why we should oppose gay marriage.  He makes some interesting claims, but here’s the assertions that he tries to back up with mostly epidemiological studies:
  1. Traditional marriage is beneficial to the public welfare.
  2. Homosexual behavior is destructive to the public welfare.
  3. The law is a great teacher; it encourages or discourages behavior and attitudes.
  4. Legalization of homosexual marriage would encourage more homosexual behavior, which is inherently destructive. It also would weaken the perceived importance of traditional marriage and its parenting role, thereby resulting in further destruction of the family and society itself.
  5. The law should endorse behaviors that are beneficial and restrain (or certainly not endorse) behaviors that are destructive.
  6. Therefore, the law should endorse traditional marriage and it should restrain (or certainly not endorse) homosexual marriage.
His last point echoes my assertion that since homosexuality is in the moral grey zone, we should neither condemn nor condone it with legislation.  Pushing for either direction is an extremist view that hijacks government for purposes beyond its intended, limited scope.