In the recent Windsor decision, the U.S. Supreme Court ordered Congress to adopt whatever marriage definition a state adopts. Thus, a couple that is "married" under the laws of a state will get all the federal benefits of marriage. I disagree with the Court's decision, but it does not seem so awful; the decision is left up to each state.
But actually there's a huge problem. Suppose a state wants a couple in a "civil union" to be treated exactly like a couple in a "marriage", except for the word "marriage". That's essentially what Proposition 8 is all about, and other states have done the same thing. But under this Windsor decision, it seems like people in marriages will get treated very differently than people in civil unions; the former will get federal benefits but the latter will not.
This new judicially-created policy contradicts a state's decision to treat marriages and civil unions the same. The new judicially-created policy also means that states that now give equal treatment to marriages and civil unions will have a huge incentive to get rid of civil unions in favor of marriages, so that gay couples can get equal federal treatment.
What Congress ought to do is simple. Replace "marriage" in federal statutes with something like "any relationship that a state calls 'marriage' OR that a state treats as equivalent to the relationships that it calls 'marriage'."
Liberals should like this because it gives even more benefits to gay couples. Conservatives should like this, because it removes a huge incentive for states to get rid of civil unions in favor of marriages. And the Supreme Court should like this, because it means the federal government would be treating couples more like each state wants them to be treated.
I agreed with the dissenters in the Windsor case. But Congress can now take action to make the majority opinion more tolerable. If Congress instead sits on its hands and does nothing, then I think you will see a rapid nationwide adoption of gay "marriage", either by states' choice or by judicial edicts or by both. That would be unfortunate, because I don't think the American people really want to be forced to call a relationship a "marriage" that has substantial differences from the usual marriages, but at the same time people want to be fair and evenhanded.
True, single people like me would get the shaft whether or not Congress does as I've suggested; we single people just don't fit into the narrative of bigotry and oppression that the Supreme Court has spun. Them's the breaks. But if we take the Supreme Court at its word, then states remain free to treat people in gay relationships like single people, or like straight married couples, or any way else that states think is apt.