My Daily column this week looks at Mitt Romney’s very good May, and wonders what if this is as good as it gets?

And in the Weekly Standard Newsletter (which you should sign up for, because it’s all-new essays, not just repackaged content) I put forth a modest proposal for gay marriage. Excerpt:

Conservatives have two big practical concerns about gay marriage. (We’re going to put aside all of the philosophical objections and worries about unintended consequences for the moment.) The practical concerns are these: (1) Would the expansion of the marriage contract stop with homosexual couples, or would it be pushed further? And (2) Would religious institutions be compelled to recognize and/or perform gay marriages?

You can tell a lot about the gay marriage debate by the way proponents of gay marriage address these concerns. Instead of trying to reassure conservatives on these points so as to persuade them, the people who want same sex marriage ignore or mock them.

For instance, in New Hampshire, Rick Santorum suggested that if gay marriage were made legal, it was difficult to say why polygamy wouldn’t be next. He was jeered and ridiculed for linking the two.

But where did Santorum get the “crazy” idea in the first place? It turns out that there’s an entire movement of gay marriage proponents who have already started advancing the legal ball toward polygamy. In 2006, a group of liberal academics, legal scholars, and activists got together to think about what the future of marriage should be after same sex marriage was legalized. The result of their work was the Beyond Marriage Project, which proposes a radical expansion of the marriage contract so that, among other things, there would be legal recognition for “households in which there is more than one conjugal partner.” Which is the polite way of saying “polygamy.”

Since the Beyond Marriage Project launched, hundreds of liberal thinkers have become signatories to its proposal. Some of these supporters are folks from organizations with names like “Unitarian Universalists for Polyamory Awareness.” But many of them are quite respectable. One of them, for instance, is Chai Feldblum—an impressive legal theorist from Georgetown Law School who is now serving as the head of President Obama’s Equal Employment Opportunity Commission. Feldblum isn’t crazy in the least. What would be crazy would be to dismiss her thinking as part of some fringe.

So conservatives aren’t making this stuff up. Now maybe most of today’s gay marriage advocates would never support polygamy. But maybe not. Forty years ago, lots of liberals said the same thing about the notion of “marriage for gays.” And look where we are now.

Then there’s the matter of religious freedom. If same sex marriage were legalized, would churches that reject the idea be forced to recognize it—or even perform gay marriages? No one really knows. Yet consider this: Supporters of same sex marriage often argue for it by saying that years from now, we’ll look back on this moment as a civil rights episode equivalent to the rebellion against anti-miscegenation laws.

Well, once anti-miscegenation laws were repealed, all of America had to comply—even non-profit groups. The most prominent institution that still recently banned interracial dating was Bob Jones University. And in order to keep this ban in place, Bob Jones had to give up its non-profit tax-exempt status. (The university dropped the policy in 2000.) If America legalizes same sex marriage and, say, the Catholic church refuses to marry gay couples, would the church lose its tax-exempt status, too? Most supporters of gay marriage insist that it wouldn’t, but the truth is, no one really knows.

There is an obvious solution. If gay marriage supporters really believe that the redefining of marriage will stop at same sex couples, and that institutions of civil society will be allowed to retain their own views on the subject, then they could propose a constitutional amendment.

In 2002, a group of conservative Democrats and Republicans proposed a Federal Marriage Amendment, the goal of which was to build a constitutional ban on same sex marriage. There’s no reason supporters of gay marriage couldn’t craft their own amendment. A “Federal Gay Marriage Amendment” could set out three principles: (1) that marriage is between two, and only two, individuals; (2) these individuals can be of any sex; (3) religious groups retain a conscience exemption in the recognition of any form of marriage.

Such an amendment would provide a constitutional guarantee to gay couples while also taking polygamy off the table and providing protection for churches. Many, though not all, conservatives might be inclined to support it. (As a bonus, it would take the issue out of the courts—finally giving gay marriage popular legitimacy, too.)

So ask yourself: If their goal isn’t to keep pushing the boundaries of marriage and to coerce religious institutions into agreement, then why haven’t advocates of same sex marriage argued for such an amendment?

There are only two answers. The first is that they just haven’t gotten around to it—and that if public opinion continues to evolve in their direction they eventually will.

The second is that, despite their reassurances to the contrary, the mere legalization of same sex marriage is not the movement’s final destination but just a weigh station on the path to even bigger changes.