When it comes to civil union, two consenting adults should be free to live together and have all the insurance, inheritance, medical decision making and tax benifits/penalties as anyone else.
But, when it comes to marriage, what is the difference between civil union and marriage? The difference is the right and responsibility to genetically create offsping and responsibility to raise those offspring.
The biological production of children is exactly why marriage is a legal issue. Children that are not provided for become a liability to the community.
Additionally to legal and spiritual concerns are serious biological issues here. There already has been the development of technology for a male-male or female-female couple to biologically reproduce using a donor egg from a third person.
Marriage is a civil rights issue but not of the same-gender couple but the unborn child who has the right to receive a genetic inheritance from both a male and female.
The consequences of non-random genenetic recombination is the creation of trans-human breeds. Phenotypically trans-human breeds may display more variation, but genetically they will have less diversity and be less fit and like the domesticated dog, will never again have the strength to stand up against the wolf.
This whole same-gender issue is about demoralizing America and taking a big eugenics step towards enslaving the human race and creating a master class and servant class. (see Georgia Guidestones)