REALITY TEST: The legality, and Constitutionality, of something is predicated on demonstrative harm to society. Marriage is a STATE defined concept. It is something conducted between individuals of different Religions (religion being a matter of choice), and different Races (race being a reality of birth). Be it because of genetics or personal preference, the state has no basis for denial of the rights granted to those who enter into marriage -- tax status, survivorship, pension rights, etc. The state can define what marriage is -- monogamous or polygamous -- and it can set an age of consent which reflects sexual consent. But, having granted the right of sexual consent, it cannot restrict marriage between consenting parties who seek to avail themselves of the legal constructs which accompany a marriage certificate.
Homophobes, and the unthinking, argue same-sex marriage opens the door to alternations in the age of consent -- obvious nonsense and stupidity. However, denial of marriage to those who are legally entitled to consensual sexual activity can and will open the door to denial based on race, religion, original nationality, culture or any other "differences" or "similarities" between parties.
The only valid basis for the court to void same-sex marriage work be clear irreparable harm to society, or to one of the individuals involved (as occurs in child molestation). Simply put, a negative finding would destroy multiple aspects of the social fabric.