Regardless of what the Supreme Court decides this week, there is no reasonable basis—historical, scientific, or constitutional—for granting marital rights to any couple other than a man and a woman.
Let's start with history. For thousands of years and in every civilization, marriage has been between men and women. By contrast, the legal bonding of same-gender couples is very scarce—in fact, I'm not sure there are any historical examples. (The Sacred Band of Thebes are sort of an example, but since they were slaughtered to the last man by the Macedonians, we might not want to follow it.)
As for science, the social sciences pretty strongly show that children do best when they live with both biological parents and the parents are married. Google the studies if you like. The research clearly reinforces the historical experience and intuition.
So if there's controversy at all, we're really talking about legal and constitutional rights.
In America, marriage is a specific legal status, and entering into marriage is considered a civil right. But no right is totally unlimited. In fact, public policy concerns shape all of our rights. (The famous statement about shouting "Fire" in a crowded theater is just one example.) Since America's beginnings, marriage has been administered by the states and limited by age (children may not marry) and blood relationship (certain relatives may not marry). Marriage as a legal status exists because society as a whole has an interest in the stability of relationships and the raising of children.
Marriage was understood as the union of man and woman in every state from the time of America's founding, as it had been for thousands of years previously in the cultures from which our nation drew (English, German, Celtic, Greek, Roman, and Hebrew). That specific definition of marriage was a fundamental assumption of the English common law, the backbone of our legal tradition. The US Constitution does not give any organ of government the power to redefine marriage, and I don't think such a power lies in the 10th Amendment's reservation to the states. Even if such a power exists, the people of California were well within their civil rights to choose not to exercise it, and the 14th Amendment does not empower courts to force otherwise. Similarly, in passing DOMA, the Congress acted well within its area of responsibility.
So-called "gay marriage", like its equally oxymoronic predecessors "free love" and "no-fault divorce", is an attempt to disregard human nature and the lessons of history. And we know how well that always turns out.