As early as 2006, for example, in the statement "Beyond Same-Sex Marriage," prominent figures including Cornel West, Gloria Steinem, Barbara Ehrenreich, and Judith Butler called for the legal recognition of polyamorous households "in which there is more than one conjugal partner." Having jettisoned a view of marriage as procreative in nature (and so necessarily a union of a man and a woman) marriage revisionists have also given up on the idea that marriage should last a lifetime or be limited to one partner.

And just before the New York legislature voted to approve same-sex civil marriage, Katherine M. Franke-a signatory of "Beyond Same-Sex Marriage-hailed the New York vote with an op-ed for the New York Times that called for putting "non-marital ways of loving" on an equal footing with marriage and same-sex civil marriage. Franke, who is the director of the Center for Gender and Sexuality Law at Columbia Law School, argued that non-marital arrangements "far exceed, and often improve on, the narrow, legal definition of marriage."

So there is no reason to think that same-sex civil marriage is the end of the road for marriage revisionists. Even if it were, it would be far from clear just what supporters of the new law mean when they speak of marriage. The New York Times reported in 2010 on a study of 556 gay couples that found that nearly 50 percent of the couples were in open relationships. As many advocates acknowledge, exclusivity is just not part of the equation.

Of course, same-sex-attracted Americans are hardly the only ones who seek to enjoy the benefits of marriage without being disciplined by its very real demands. While most Americans still view marriage as an exclusive relationship, it is by no means seen as permanent. The rise of no-fault divorce and casual cohabitation paved the way for same-sex civil marriage by making marriage a forum for the expression of feeling-what one advocate has called "maximal experiential union"-rather than a binding tie oriented to the rearing of children and the pursuit of virtue.

When I speak to my friends who favor same-sex civil marriage, they tend to rely on two false trump cards to support their view that the struggle for marriage is over. First, they claim that supporters of marriage have run out of arguments for their position. This argument ignores the fact that marriage supporters have repeatedly explained the connection between procreation, permanence, and exclusivity-most notably in "What Is Marriage?" an article by Sherif Girgis, Ryan T. Anderson, and Robert P. George for the Harvard Journal of Law and Public Policy.

The three co-authors ably defended their views in extended exchanges with Andrew Koppelman and Kenji Yoshino, in Public Discourse, and with Jason Lee Steorts, in National Review. Reading these debates gives the opposite impression of the usual claim: It is the marriage revisionists who lack arguments and must instead rely on hollow accusations of bigotry and bad faith.

The second trump card is the claim that same-sex civil marriage is inevitable. This is a view shared, strangely enough, by both optimistic progressives and pessimistic conservatives. The former think the world is getting better and the latter think the world is getting worse, but both view the permanent adoption of same-sex civil marriage as a foregone conclusion.

I reject that. There is nothing inevitable about same-sex civil marriage, nor is there any reason to think it invincible once it is codified in law. History is neither a straight line of progress nor is it (as President Obama would have it) an arc that bends toward justice. It is, if anything, a zig-zag that moves unpredictably, first in one direction and then another. Even those who make every effort to get on the "right side of history" will find that it turns on them.

The reason for this is that humans are free, really free, not only to pass silly laws but also to grow to regret them. As the Colombian aphorist Nicolás Gómez Dávila wrote, 'History is indeed the history of freedom-not of an essence "Freedom," but of free human acts and their unforeseeable consequences.'

Few could have predicted twenty years ago today's drastic revisions in marriage, and no one can predict twenty years from now whether those changes will be further entrenched or on their way to being undone. This is the stuff of history-not a long certain march but an uncertain muddling through.

The vote in New York did not represent the end of a movement, but it did, just maybe, mark the beginning of one. In the wake of the vote I heard from many friends, most of them young, who took the Empire State vote as a call to rebuild a vibrant marriage culture, both in their religious communities and in society at large.

Already there is a movement of young Catholics, Evangelicals, Jews, and Mormons (some of whom I have the privilege of knowing) to revisit the assumptions embedded in no-fault divorce and same-sex civil marriage. These young people-including the young leaders of the Love and Fidelity Network, Princeton University's Anscombe Society, and Harvard University's True Love Revolution-believe that they are required in charity to work to reclaim a marriage culture that balances the desires of adults alongside the needs of children for a mother and father.

Of course, it is far from certain how this movement will proceed or what success it will have, but I draw hope from its youth, intelligence, and good nature. Like the pro-life movement before it, it will be motivated by a concern for unseen victims and a respect for the self-evident facts of nature. I firmly believe that the witness of these brave young leaders will give many more the courage to join them.

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Matthew Schmitz is Deputy Editor of First Things.