The most common argument of all against gay marriage is the procreation argument. It can be stated this way: "Procreation is indispensable to human survival. Marriage is for procreation, and procreation should occur within marriage. Procreation is the one important attribute of marriage that supplies the male-female definition. Gay couples can't procreate as a couple, so gay couples shouldn't be allowed to marry."

The argument over procreation has generated a back-and-forth between advocates and opponents of gay marriage that follows a familiar and somewhat tedious pattern. It goes something like this:

Thrust (by gay-marriage opponents): Marriage is for procreation. Gay couples can't procreate. So gay couples should not be allowed to marry.

Parry (by gay-marriage advocates): But procreation has never been required for marriage, so the premise that "Marriage is for procreation" is wrong, or at least incomplete. Sterile couples, old couples, and couples who simply don't want to procreate are all allowed to marry. Nobody objects to their marriages, so nobody should on this ground object to same-sex marriages.

The parry by gay-marriage advocates is sometimes called the "sterility objection." Let's take the argument beyond this standard thrust-and-parry.

1. The sterility objection to the procreation argument: two responses and counter-responses.

The procreationists have a couple of fairly standard responses to the sterility objection.

First, they say that laws are made for the general rule, not the exceptions. Most opposite-sex couples can reproduce, but no gay couple can. Second, they argue that the failure to require married couples to procreate is only a concession to the impracticality and intrusiveness of imposing an actual procreation requirement. It is not an abandonment of the procreation principle itself. We need no intrusive test to know same-sex couples can't reproduce (as a couple), the procreationists observe.

The first response to the sterility objection -- that laws are made for the general rule — is an evasion. Laws do often state general rules, and they are often over- or under-inclusive in some way. But laws also provide exceptions where appropriate and just, where some policy purpose is served by the exception. Gay marriage, like non-procreative straight marriage, might be a good policy exception to the procreationists' rule that marriage exists for procreation. Whether gay marriage is a good exception to the asserted general rule that marriage is about procreation depends upon arguments extrinsic to the procreation argument, e.g., whether encouraging stable gay coupling through marriage would benefit gay families and society. (Remember, I'm not making a constitutional argument about whether the government should, as a constitutional matter, be able to make marriage laws that are not narrowly tailored to the state's claimed interests.)

The second response to the sterility objection -- that a procreation requirement would be unduly intrusive -- is equally unavailing. If we were serious about the procreationist project -- that is, if we were serious that procreation is the only public interest in marriage -- we could require prospective married couples to sign an affidavit stating that they are able to procreate and intend to procreate. (We could entirely bar from marriage elderly couples beyond a certain age.) If, say, in ten years they had not procreated we could presume they are either unable or unwilling to do so and could dissolve their union as incapable of satisfying the only public purpose of marriage. That system would not require invasive fertility examinations.

Yet we would never require opposite-sex couples to fill out such a fertility form. I think most people would scoff at the idea, and rightly so. They would even think it's cruel, especially perhaps to elderly couples. But why? They would scoff at the idea because marriage today is not understood to be essentially about procreation, although procreation-within-marriage is important. Marriage is understood today to have other important public functions and purposes -- including providing the individualistic and communitarian benefits I have outlined.

Here the procreation argument suffers an experiential flaw; it is like an argument from another world, not the world we inhabit. In the world we inhabit, procreation is an important but not essential attribute of the public institution of marriage.

This may also expose a potential political flaw in the procreation argument: by repeatedly emphasizing that the only public purpose of marriage is procreation, opponents of gay marriage run the risk of demeaning the many married couples for whom children are either unwanted or impossible. Yet their marriages are celebrated, not simply tolerated and certainly not disdained.

Further, this second response to the sterility objection suggests that the general rule of procreation must bend (1) to the overriding needs and interests of society to help individuals settle down and (2) to the interests of the couples unable or unwilling to live by the procreation purpose. If that exception exists for non-procreative straight couples, why not for non-procreative gay couples? If it would be cruel or pointless to deny them marriage, why not gay couples? If there is an answer to this question, it cannot be found in the procreation argument.

2. Practical consequences to procreation of admitting gay couples to marriage.

Even if the procreation argument seems logically weak, are there practical consequences to human procreation of admitting gay couples to marriage? I can think of two possible fears. One fear is that procreation itself would slow down, perhaps below the "replacement rate," the level at which humans must reproduce in order to stay ahead of deaths. This slowdown would eventually imperil the species. The other fear is that, as the connection between marriage and procreation is loosened, procreation may increasingly occur outside of marriage. Both could happen at once, and both would be bad.

But neither of these consequences is very plausible. Start with the fear of a population implosion. How would allowing gay couples to wed cause a decline in reproduction rates? It's not clear why straight couples would stop procreating, or even procreate less, if gay couples could marry. The factors driving people to reproduce — the needs for love and to love another, the instinct to propagate one's genes, religious obligations — would all still exist if Adam and Steve could marry. If Western civilization is truly facing a population implosion, as some suggest, that is attributable to many complex factors that are already in play (like great wealth and better health in old age), long before gay marriage was even a twinkle in Andrew Sullivan's eye.

Here's one possible mechanism arising from gay marriage that might lead to population decline. Professor Douglas Kmiec, quoting Robert Bork, has argued that gay marriage "'will lead to an increase in the number of homosexuals.'" Kmiec, The Procreative Argument, 32 Hastings L. Q. 653, 661. Perhaps, the procreationist might conjecture, there are some "waverers" -- people who stand somewhere between homosexuality and heterosexuality -- who will be brought toward more homosexual behavior by the stigma-easing effect of permitting gay marriage. More homosexuals means less procreation, the theory goes.

There's been a lot of research on sexual orientation in the past few decades, and I've never seen good evidence for the waverers theory. Sexual orientation, whatever its causes, appears for the vast majority of people to be unchosen and at least strongly resistant to change based on incentives in public policy. The idea that the incidence of homosexuality in a society varies with the degree of legal repression or acceptance shown toward homosexuality has no empirical support. Richard Posner, Sex and Reason, at 163, 296-97. Homosexual preference appears to be no more common in tolerant societies than in repressive societies. Id. at 296.

But even assuming there are waverers, the idea that they would contribute meaningfully to a population implosion is not plausible. Remember, for this theory to be correct, there need not only to be waverers but enough of them brought into homosexuality by the gay marriage to make any real difference in rates of reproduction. There is no evidence to support the idea that they exist in such substantial numbers.

Not only that, these waverers would have to more than offset the gains in reproduction from allowing gay marriage. After gay marriage is allowed, closeted homosexuals will be less likely to enter unhappy and unstable marriages with partners of the opposite sex. This will free up their heterosexual spouses to seek marriageable partners with whom they can procreate and form more lasting relationships. The resulting reduction in the number of such unstable marriages should be good for procreation rates and good for marriage as a whole, not bad. But this, too, will not be a large enough number to affect reproduction rates.

It's also not clear why gay marriage would drive more straight couples to reproduce outside of marriage. The legal and social-reinforcement benefits of marital procreation would still be available to them, after all. The problems of non-marital procreation would still be there to discourage it. (Maggie has a theory about this, which I'll address in the next post.)

But fortunately we do not have to guess at the probability of these cataclysmic consequences. We already have much experience with a world in which there is no requirement to procreate within marriage. No couple has ever been required to procreate in order to marry. No couple has ever even been required to be able to procreate in order to marry. Sterile couples and old couples can marry. Couples physically able to procreate but who do not want to procreate can get married.

These non-procreative categories of childless married couples are already a larger segment of the married population than the small number of gay married couples would be. Everybody knows married couples who can't or won't have children. Yet despite their inherent or explicit rejection of the putative marital duty to procreate, and despite the fact that we nevertheless let these non-procreative straight couples wed with abandon, humans continue to procreate and marriage continues to be the normative situs for procreation.

Nobody blames non-procreative married straight couples for the alleged population implosion; and nobody blames them for illegitimacy rates. Why should non-procreative gay couples, once allowed to wed, get the blame for these phenomena?

So here's where we are: millions of existing married opposite-sex couples are just as non-procreative as any married gay couple would ever be, yet gay couples are to be denied marriage because they are non-procreative.

Does Maggie have an answer for why this different treatment might be justified? I'll address Maggie's argument against gay marriage in the next post later today.