It has been noted here and elsewhere that many of the arguments used by the Right against gay marriage were at one time employed to fight against interracial marriage, which was illegal in some parts of the USA until 1967. Some history of the similarities between arguments against gay and interracial marriage can be found here. Specifically:

But it soon became apparent that Reconstruction would not survive long enough to become a turning point in the history of miscegenation law. As Reconstruction collapsed in the late 1870s, legislators, policymakers, and, above all, judges began to marshal the arguments they needed to justify the reinstatement--and subsequent expansion--of miscegenation law. Here are four of the arguments they used: First, judges claimed that marriage belonged under the control of the states rather than the federal government. Second, they began to define and label all interracial relationships (even longstanding, deeply committed ones) as illicit sex rather than marriage. Third, they insisted that interracial marriage was contrary to God's will, and Fourth, they declared, over and over again, that interracial marriage was somehow "unnatural."

In the event that you are confronted (or confronting) someone who is opposed to any form of gay marriage rights, you might consider pointing out that the main arguments used against gay marriage were successfully employed against interracial marriage. It probably won't make an immediate impression, but hopefully it will start people thinking about just how far we've come in terms of civil rights, and how far we still have to go.