How will the Supreme Court rule in this week's gay marriage cases? I have no idea. What I do know is that the outcome almost doesn't matter. One way or another, gay marriage will be legal throughout the country before long.

That's not the riskiest prediction. Plenty of pundits have said the same thing based on the stunningly rapid shift of public opinion on the issue. But public opinion can be fickle. How do we know that current trends will continue and that a backlash against gay marriage isn't right around the corner? Because even the best arguments employed by its smartest opponents are utterly unconvincing.

To be clear, I'm not talking about the explicitly religious case against gay marriage. Arguments based on orthodox Catholic, evangelical Protestant, Orthodox Jewish, or Mormon premises — premises grounded in the revelations, scriptures, and traditions of particular faith communities — are often perfectly valid. It's just that our constitutional order doesn't rest on those premises, and members of those communities lack the numbers to impose their views on the country as a whole through majority vote.

What I mean are the arguments advanced by those opponents of gay marriage who claim to have reason on their side — who wish to persuade citizens of goodwill regardless of their religious commitments (or lack of commitments). Foremost among these opponents is Robert P. George of Princeton University, lead author of an amicus brief filed with the Supreme Court. No critic of gay marriage has gone further in claiming that reason alone can tell us to reject gay marriage — and no critic has done more to demonstrate (inadvertently) how deeply confused the case against gay marriage really is.

George and his co-authors Sherif Girgis and Ryan T. Anderson make the following argument: "Our civilization" has univocally defined marriage as a "conjugal union" between one man and one woman — that is, a union between two people that is oriented to the goal of producing children. Whether or not a particular male-female couple can produce a child is irrelevant. In cases of infertility due to medical defect or advanced age on the part of one or both members of the marriage, the union falls short of reaching its goal but remains oriented to that goal nonetheless. (The union would produce a child if the bodies of both members were functioning as they should.)

Advocates of gay marriage, by contrast, seek to promulgate an alternative — a "revisionist" — definition of marriage, one based not on producing children but on "emotional fulfillment, without any inherent connections to bodily union or procreation and family life." ("Inherent" does a lot of work in that sentence, since gay couples can and do adopt children and devote themselves to family life. But because such couples can't produce the children themselves, their union remains, by George's definition, a non-procreative partnership.) This revisionist, non-procreative form of marriage would detach the institution from ideals of "permanence and exclusivity" that flow from child-rearing. That is, once couples cease viewing their union as oriented to the goal of producing children, divorce and infidelity will become commonplace. And since society has a stake in encouraging stable families, advocates of gay marriage must not be allowed to prevail.

Any number of objections could be raised against this line of argument. (Is it really true, for example, that "our civilization" has affirmed a single definition of marriage?) But I'm primarily interested in focusing on its most decisive weakness — which is that it gets a crucial chain of causality exactly backwards. Permitting gay marriage will not lead Americans to stop thinking of marriage as a conjugal union. Quite the reverse: Gay marriage has come to be widely accepted because our society stopped thinking of marriage as a conjugal union decades ago.

Between five and six decades ago, to be precise. That's when the birth control pill — first made available to consumers for the treatment of menstrual disorders in 1957 and approved by the FDA for contraceptive use three years later — began to transform sexual relationships, and hence marriage, in the United States. Once pregnancy was decoupled from intercourse, pre-marital sex became far more common, which removed one powerful incentive to marry young (or marry at all). It likewise became far more common for newlyweds to give themselves an extended childless honeymoon (with some couples choosing never to have kids).

In all of these ways, and many more, the widespread availability of contraception transformed marriage from a conjugal union into a relationship based to a considerable degree on the emotional and sexual fulfillment of its members — with childrearing often, though not always, a part of the equation. And it is because same-sex couples are obviously just as capable as heterosexual couples of forming relationships based on emotional and sexual fulfillment that gay marriage has come to be accepted so widely and so quickly in our culture. (If marriage were still considered a conjugal union, the idea of gay marriage could never have gained the support it currently enjoys. On the contrary, it would be considered ridiculous — as it remains today among members of religious groups that continue to affirm more traditional, conjugal views of marriage.)

George and his co-authors may well be right that the widespread adoption of a non-conjugal view of marriage leads to negative social consequences, including explosions in rates of divorce and out-of-wedlock births. But that's an argument against contraception, not gay marriage.

America's understanding of marriage changed decades ago, the outcome of that change is our settled custom, and though the demand for gay marriage might have been unthinkable before the change, it is hard to see how giving in to that demand will make much of a difference now. Most Americans intuitively understand this. Which is why even the most strenuous efforts of the most intellectually formidable opponents of gay marriage are bound to fail.