Dr. Robert P. George makes the case in the Public Discourse.

Excerpt:

Since most liberals and even some conservatives, it seems, apparently have no understanding at all of the conjugal conception of marriage as a one-flesh union—not even enough of a grasp to consciously consider and reject it—they uncritically conceive marriage as sexual-romantic domestic partnership, as if it just couldn’t possibly be anything else. This is despite the fact that the conjugal conception has historically been embodied in our marriage laws, and explains their content (not just the requirement of spousal sexual complementarity, but also rules concerning consummation and annulability, norms of monogamy and sexual exclusivity, and the pledge of permanence of commitment) in ways that the sexual-romantic domestic partnership conception simply cannot. Still, having adopted the sexual-romantic domestic partnership idea, and seeing no alternative possible conception of marriage, they assume—and it is just that, an assumption, and a gratuitous one—that no actual reason exists for regarding sexual reproductive complementarity as integral to marriage. After all, two men or two women can have a romantic interest in each other, live together in a sexual partnership, care for each other, and so forth. So why can’t they be married? Those who think otherwise, having no rational basis, discriminate invidiously.

[…]Thus, advocates of redefinition are increasingly open in saying that they do not see these disputes about sex and marriage as honest disagreements among reasonable people of goodwill. They are, rather, battles between the forces of reason, enlightenment, and equality—those who would “expand the circle of inclusion”—on one side, and those of ignorance, bigotry, and discrimination—those who would exclude people out of “animus”—on the other. The “excluders” are to be treated just as racists are treated—since they are the equivalent of racists. Of course, we (in the United States, at least) don’t put racists in jail for expressing their opinions—we respect the First Amendment; but we don’t hesitate to stigmatize them and impose various forms of social and even civil disability upon them and their institutions. In the name of “marriage equality” and “non-discrimination,” liberty—especially religious liberty and the liberty of conscience—and genuine equality are undermined.

The fundamental error made by some supporters of conjugal marriage was and is, I believe, to imagine that a grand bargain could be struck with their opponents: “We will accept the legal redefinition of marriage; you will respect our right to act on our consciences without penalty, discrimination, or civil disabilities of any type. Same-sex partners will get marriage licenses, but no one will be forced for any reason to recognize those marriages or suffer discrimination or disabilities for declining to recognize them.” There was never any hope of such a bargain being accepted. Perhaps parts of such a bargain would be accepted by liberal forces temporarily for strategic or tactical reasons, as part of the political project of getting marriage redefined; but guarantees of religious liberty and non-discrimination for people who cannot in conscience accept same-sex marriage could then be eroded and eventually removed. After all, “full equality” requires that no quarter be given to the “bigots” who want to engage in “discrimination” (people with a “separate but equal” mindset) in the name of their retrograde religious beliefs. “Dignitarian” harm must be opposed as resolutely as more palpable forms of harm.

[…][T]here is, in my opinion, no chance—no chance—of persuading champions of sexual liberation (and it should be clear by now that this is the cause they serve), that they should respect, or permit the law to respect, the conscience rights of those with whom they disagree. Look at it from their point of view: Why should we permit “full equality” to be trumped by bigotry? Why should we respect religions and religious institutions that are “incubators of homophobia”? Bigotry, religiously based or not, must be smashed and eradicated. The law should certainly not give it recognition or lend it any standing or dignity.

The lesson, it seems to me, for those of us who believe that the conjugal conception of marriage is true and good, and who wish to protect the rights of our faithful and of our institutions to honor that belief in carrying out their vocations and missions, is that there is no alternative to winning the battle in the public square over the legal definition of marriage. The “grand bargain” is an illusion we should dismiss from our minds.

You can read about some examples of attacks against proponents of traditional marriage in my secular case against same-sex marriage.