These were fascinating comments, to which I cannot do full justice.
One issue that a number of comments raise is whether honosexuality is truly genetic or otherwise innate, or is a choice. Some distinctions will help to focus the issue. First, there is a fundamental difference between homosexual behavior and homosexual orientation. Many heterosexuals engage in homosexual behavior when there is no heterosexual alternative open to them--e.g., men in prisons or on naval vessels (before they were integrated), teenage boys in all-male boarding schools, and young men in Mediterranean cultures where marriage is late and women are largely unavailable for sex outside of marriage. I discuss this "opportunistic" homosexuality at length in my 1995 book Sex and Reason. There is also opportunistic heterosexuality--notably, men and women who enter heterosexual marriages to have children and/or conceal their homosexuality. I can't see a good reason for encouraging either kind of behavior.
Concerning homosexual orientation as distinct from behavior, there is an important distinction between genetic and innate determinants. One could be born possessing a particular trait because of one's genes, or because of some accident during pregnancy. In either case, the trait would not be "chosen." In the case of homosexual orientation, there is a fair amount of evidence that it is genetic; I discuss this in my book; also, a recent study reported in the New York Times actually found a gene that causes homosexual behavior in fruit flies. The evidence that homosexual orientation is, if not genetic, certainly innate is more diffuse but cumulatively persuasive, and includes the fact that homosexuality seems to crop up in all human cultures, including ones that reprobate it, and the almost complete failure of efforts to "cure" homosexuality despite the strong incentives of those who submit to such treatments. The "cure" when successful consists not of acquiring heterosexual orientation but of suppressing homosexual behavior. If homosexual orientation were simply a "bad habit," it could be broken more easily than cigarette smoking, an addiction that has a physical as well as a psychological dimension; no one thinks it can be.
Once the innate character of homosexual orientation is accepted, the way is paved for analogizing, as a number of the comments do, the claim of equality for homosexuals to the claim of equality for members of racial minorities--the latter a claim almost universally accepted in this society. I accept the analogy, and of course also accept that the Supreme Court in Loving v. Virginia held that it is unconstitutonal for government to forbid interracial marriage. My reasons for nevertheless opposing courts' ruling that it is unconstitutional to forbid gay marriage are twofold. First, the courts would benefit from a period in which experience with gay marriage in one or more states and several foreign nations, together with growing experience with civil unions, would lay a solider empirical basis than exists now for assessing the consequences of gay marriage. There is value in social experiments, and hence in not terminating them prematurely. (Compare the bad practice of terminating clinical drug trials as soon as there is some, but often incomplete and inconclusive, evidence that the drug being tested does have some therapeutic value, which the placebo administered to the control group does not.)
Second, and at the risk of seeming to take a Realpolitik approach to constitutional law, I don't think it's the business of the courts to buck public opinion that is as strong as the current tide of public opinion running against gay marriage. That is the lesson of the response to Roe v. Wade, even though there is far more public support for abortion rights than for gay marriage. Because the basis in conventional legal materials for creating a constitutional right either to abortion or to gay marriage is extremely thin, opponents cannot be persuaded that the creation of these rights by courts is anything other than a political act by a tiny, unelected, unrepresentative, elite committee of lawyers.
It is true of course, as one of the comments pointed out, that Brown v. Board of Education, in declaring racial segregation in public schools illegal, outraged many Southerners, but this was understood to be the reaction of a national minority (Southern whites) selfishly motivated by a desire to maintain black people in a politically and economically subordinate, dependent position. Brown would have been unthinkable--and in my pragmatic view unsound--had the case arisen in 1900 rather than the 1950s, because in 1900 the vast majority of the American population would have considered compelled racial integration of public schools improper. Moreover, I do not think the opposition to gay marriage is primarily motivated by a desire, even an unconscious one, to subordinate homosexuals to heterosexuals, or by a fear of homosexual recruitment--though that is a factor, as I mentioned in my original posting. I think the main basis for the opposition is religious and (responding to another comment) that such opposition is different from opposition based on a scientific error. Religion is not scientific, but there is a difference between a belief that is demonstrably based on error and a belief based on a system of thought that science neither supports nor refutes. (Science cannot refute the existence of the soul, for example, because there is no scientific test that could refute it. Sophisticated religions are careful to place their key claims beyond the possibility of scientific falsification.) In a democratic society, one has to respect religious beliefs; and no reasonable theory of the meaning of the religion clauses of the First Amendment permits one to argue that religious belief cannot be permitted to influence secular law. No one supposes that punishing murder is an establishment of religion just because the Ten Commandments--a religious code--states "Thou shalt not kill."
Although I don't think that courts should force gay marriage on the society, the arguments against gay marriage do not strike me as compelling. For example, it is true that the institution of marriage is oriented toward the production and rearing of children, but there are so many childless marriages, including second or third marriages of divorcees or widows or widowers, that it would be arbitrary to forbid gay marriage on the ground that, on average, such marriages produce fewer children. And indeed, if it is correct that most gay marriages will be between lesbians, the average number of children in gay marriages may not be significantly below the norm, since most lesbians, like other women, I imagine want to have children. Nor am I aware of evidence that children raised in homosexual households are on average more maladjusted, unhappy, antisocial, etc. than the rest of us; the evidence I reviewed in Sex and Reasondid not support such a hypothesis, though the book was published a decade ago and perhaps there is now some evidence to support it. Finally, it is unclear to me that marriage is any longer heavily subsidized--there is after all the "marriage tax" to consider.
One last point. One of the comments points out correctly that civil unions do not give homosexuals the full equivalent of marriage because the federal government refuses to recognize them as marriage equivalents for purposes of social security and other federal benefits. But that is equally true of gay marriages contracted in Massachusetts: the federal government does not recognize these as "marriage" for federal-law purposes. I believe that the prospects for federal recognition of civil unions would be greater if the homosexual-rights movement dropped gay marriage and focused entirely on civil unions. Gay marriage is the red flag before the bull.