One of the less remembered footnotes of '04 was Rove and Co. getting gay marriage bans on the ballot in six swing states to push GOP turnout. That probably did more than anything else (even OH restricting voting machines in heavily Democratic Cleveland and Columbus) to re-elect Bush, but the laws were VERY narrowly written to exclude all but heterosexual religious marriages. The effect was exactly what you expected: Millions married by JPs, common law etc. woke the next day to find they had retroactively annulled their own marriages. Who knows; that might be part of why the scales have tipped the other way since.

Legislatively, nothing would be required except a law stating something like "each statutory reference to 'marriage' shall henceforth be understood to civil unions, with all pursuant legal obligations and privileges, and no bearing on or by religious rites." Such simple revisions have a long history as law changed to give women and blacks equal status they previously lacked with white men. Doing it again would be trivial, as evidenced by the fact we have already done it in most other areas.

The best thing about a revision like that described, however, is it does not say, "gay people, too," or reference any particular group, but universally covers everyone. That is ever and always the proper goal in civil rights; explicitly specifying individual groups must be "treated as equal" to others implicitly establishes they are not and leads to things like Plessy v. Ferguson. It pretends others are equal, to salve our conscience and prevent their outrage, while making clear that is only a grudging conceit behind which the same old prejudice lurks as strong as ever. It was enough to affirm all people equal back in 1776; we shamefully failed to live up to that ideal then, but it remains the ideal for which we should strive, precisely because it is universal in the way no other standard can be.