I'm not a lawyer, just an engineer. But, I have a copy of the Constitution handy and I think it is both a 10th and a 1st amendment (free exercise of religion) issue.
"The powers not delegated to the United States by the Constitution, nor prohibited to it by the States, are
reserved to the States respectively, or to the people."
To me this indicates that Gay Marriage can not be regulated by the FedGov. My opinion is that Marriage is a religious sacrament and is a 1st amendment issue that can not be regulated by either FedGov or State Gov.
To the extent that the states have an interest in the care of the young and financial security there may be cause for civil unions, but not marriage since it is a religious sacrament.
This should satisfy no one.