I am truly astonished at the number of otherwise apparently rational people who are advancing (and otherwise rational editors who are choosing to publish) the argument that gay marriage ought to be illegal because gays can't reproduce. For one thing, it's clearly not true. From the point of view of reproductive abilities, a lesbian couple is at least as capable (arguably twice as capable) of reproducing as a heterosexual couple whose male partner happens to be sterile or has had a vascectomy. So even if one accepts the premise that society has a vested interest in supporting the production of babies that does not hold up to scrutiny as grounds for opposing the marriage of lesbians.
But the premise is also clearly not true. There may have been a time when society had an interest in encouraging the production of babies, just as there was a time when it had an interest in encouraging everyone to produce (or hunt or gather) food. But those days are long gone. There is hardly a baby shortage in the world. What there is a shortage of, and what the institution of marriage was designed to foster it seems to me, is stable, loving families willing to invest the 18 years or so it takes to raise a child, and in this regard gays are just as capable as anyone else (more so if my gay friends are any indication).
I also wonder in passing how many Americans making the argument that gays shouldn't be allowed to marry because they can't produce babies simultaneously support stricter enforcement of our immigration laws. I don't have any data, but I suspect the number is large. This, I think, reveals this argument for what it truly is: a thin disguise for bigotry, because at the same time they wring their hands about the terrible shortage of babies they want to make sure that it's the right kind of babies. Mexican or Romanian babies just won't do. This mindset is no different from the one that led people to oppose interracial marriage in this country not so long ago.
As an aside, there is an interesting review of the history of homosexuality in society here. I'd take some of what he says with a grain of salt, e.g., "There is almost no evidence in any pre-Christian writer of hostility to homosexuality as such." Leviticus 18:22 says "Thou shalt not lie with mankind, as with womankind: it is abomination," which certainly sounds to me like pre-Christian hostility to homosexuality as such. But maybe that's why he hedged with "almost."
I found another interesting passage while searching for that one: Exodus 22:16 says (or at least strongly implies) that pre-marital sex is OK as long as you marry the person afterwards.