The laws before the court are the Defense of Marriage Act, passed handily by a bipartisan majority of Congress and signed by President Bill Clinton in 1996, and Proposition 8, the measure passed by California voters in 2008 enshrining the traditional definition of marriage in the state's constitution.
The Defense of Marriage Act is a modest measure. For purposes of federal programs, it defines marriage as between a man and a woman, and it says that states don't necessarily have to honor same-sex marriages from other states. This creates a flexible environment whereby the federal government recognizes the traditional understanding of marriage that still applies in more than 40 states, while any state is welcome to adopt any other definition of the institution that it sees fit.
Opponents of the law have concocted an argument against it on federalist grounds. But it is bizarre to contend that a federal law defining marriage for federal purposes is an offense against the federalist structure of American government. The law has done nothing to arrest the progress of gay marriage at the state level, where it now prevails in nine states and the District of Columbia despite the Defense of Marriage Act.
The real reason for the court to invalidate the law would be that it supposedly has no rational basis and is borne of "animus" toward gays. This is the brief against Proposition 8, which was struck down by a federal appellate court, the famously activist 9th Circuit, on grounds that it has no "legitimate reason."
In this view, the promoters of Proposition 8 came up with a definition of marriage that has stood for centuries in the West and is endorsed by every major religion simply as an imaginative way to stick it to gay people. Every serious contender in the Democratic presidential primary in 2008, including Barack Obama, supported this same definition, presumably also out of the same simmering hostility to gays.
Supporters of traditional marriage believe that the institution exists as an expression of society's interest in children being raised by their biological fathers and mothers. You can say that this understanding is dated, given what has become of marriage the past 40 years. You can say that it is too pinched, given evolving mores. You can't say it is inherently hateful.
If the Defense of Marriage Act is wrongheaded, the solution is simple and will be within reach in a few years if gay marriage continues to win converts repeal it. And there is nothing wrong with Proposition 8 that California's voters can't fix by going to the polls again.
By seeking a shortcut in the courts, supporters of gay marriage want to end debate through judicial fiat. In an amicus brief in the case, the Becket Fund for Religious Liberty points out the consequences if traditional marriage is deemed irrational. Religious people and groups objecting to same-sex marriage will "face a wave of private civil litigation under anti-discrimination laws never intended for that purpose," and they will be "penalized by state and local governments."
In other words, supporters of the exotic-sounding practice of "opposite-sex marriage" will be marginalized forevermore.