One of the great coups of the movement for same-sex marriage has been to plant the premise that it represents the inevitable future. This sense has inhibited even some who know perfectly well that marriage is by nature the union of a man and a woman. They fear that throwing themselves into the cause of opposing it is futile — worse, that it will call down the judgment of history that they were bigots.Emphasis is mine. But the error is the National Review's; both gay marriage and civil unions are becoming more popular with the public, although at a relatively slow rate.
Contrary to common perception, however, the public is not becoming markedly more favorable toward same-sex marriage. Support for same-sex marriage rose during the 1990s but seems to have frozen in place (at least according to Gallup) since the high court of Massachusetts invented a right to same-sex marriage earlier this decade.
Below is a chart comprising polling data from PollingReport.com on the gay marriage and civil unions questions, with trendlines derived via LOESS regression. This chart includes all surveys in the PollingReport.com database, except those where the respondent was given a three-pronged choice between gay marriage, civil unions and nothing.
The National Review is correct that the trend toward greater support for gay marriage and civil unions stalled out some in the wake of the Goodridge v. Dept. of Public Health decision in November, 2003. (In fact, it may have temporarily reversed itself). They are wrong, however, that gay marriage has failed to gain support since that date. Per the LOESS curves, gay marriage has gained about 8 points of support since November 2003, while civil unions have gained about 13 points of ground. Because of sampling variance and vagaries of question wording from survey to survey, the trend isn't always perfectly smooth. But the overall trend is fairly manifest; the rate of increase in support for gay marriage and civil unions has if anything accelerated since the Massachusetts decision.
This does not mean that gay marriage is "inevitable". On some "cultural" issues such as marijuana legalization, there have been periods of years at a time when the "liberal" position was losing ground (although marijuana legalization has since regained support). The public's views on abortion, meanwhile, have been stuck at more or less the same numbers -- with a narrow majority/plurality taking the pro-choice position -- for literally decades.
Support for gay marriage, however, is strongly generational. In a CBS news poll conducted last month, 64 percent of voters aged 18-45 supported either gay marriage or civil unions, but only 45 percent of voters aged 65 and up did. Civil unions have already achieved the support of an outright majority of Americans, and as those older voters are replaced by younger ones, the smart money is that gay marriage will reach majority status too at some point in the 2010's.