There's a difference between policies that "promote" and policies that "prohibit" or "enforce." You speak of heterosexual marriage as having, on balance, a more quantifiable and clear-cut economic and social benefit to society at large. You also say that allowing governments to both validate and enhance such partnerships promotes their creation and facilitates their success. True. I agree.
At the same time, there is a strong argument to be made that such social policies should never "enforce" nor "require" child-bearing. Simply put, civil benefits that accrue to heterosexual marriage simply create the conditions for success, but neither forbid the dissolution of these partnerships nor enforce their supposed policy goal, to wit, the children. Oh, the children. So in the end, while it is in the interest of those couples wishing to start a family to have their union recognized by the government, at the end of the day, that recognition imposes no positive procreative obligations on the couple. Still agreed? I think so.
So we've seen that the government is (rightly) willing to elevate freedom of choice within marital relationships over compulsion to support the greater good--in other words a society that invests in its material survival willingly but decreasingly is preferable to one that invests increasingly but by force. Therefore, enforcement of child-rearing shall play no role in laws governing marriage.
But what about prohibitions? For you, homosexual marriage, which by simple mechanics cannot, by itself, produce offspring, cannot fall within the stated policy goals of legally-recognized marriage. Therefore, the argument goes, it ought to be prohibited. Some take the argument a step further, and say that not only does homosexual marriage fail to meet the threshold policy requirement for legal marriage, it actively undermines the policy goal by (presumably) implying that children are not the true end of such legal partnerships. The result? Less babies.
The latter part of this argument fails somewhat miserably. I cannot adduce a single heterosexual married individual who has ever looked to the behavior of a homosexual couple as exemplars of normative procreative behavior. Outside of personal experience (which is weak, I know), I still await the day someone establishes a link between government-recognized marriage between homosexuals and a decrease in the willingness of heterosexual married couples to produce offspring. Good luck with that. Baby-making has been on the decline for a while now, a consequence, of a number of things, among them increased secularization, the decline of agrarianism, increased presence of women in the workforce, and an increase in marriages at more advanced ages. (On a sidenote, if anything has affected the real bottom line--that is, happy, stable, economically sound family units--it is divorce.) If homosexuals not marrying in California pursuant to Prop 8 is shown to increase baby production, I'll have to re-think my position.
On the other hand, the first part of the argument--that homosexual marriage simply doesn't fall within the policy goals of government-authorized marriage--is more compelling. Here, I would tend to fall within the camp that advocates a more expansive view of marriage. (And no, marriage in its current heterosexual incarnation is not representative of how marriages have been conducted throughout human history and therefore normative. See "Women--Property." Marriage is very much a fungible concept.) The reduction of marital partnerships to lines of production is fine for a formalistic argument about the survival of the species, or even, more to the point, of the society, but if reading the great books of the last three millennia (and being a human being) has taught me anything, it's that our species has a curious disregard for simple survival. We've also already seen that governments not only have an interest in legally recognizing heterosexual marriage, but also an interest in NOT DISSOLVING MARRIAGES THAT FAIL TO PRODUCE CHILDREN!!! Why ever would the government refrain from dissolving partnerhsips that fail to produce children? Could it be, perhaps, that the freedom to choose to have children is a superlative policy consideration? Could it also be that the human experience is enriched, enlivened and enhanced by the action of directing one's love towards another person for the duration of his/her life?
That's right, we're back in that nebulous territory of "happiness." And "love." In all seriousness, though, absent an argument that can conclusively show that homosexual marriage actively undermines the procreative ends of heterosexual marriage, and absent an argument that homosexual marriage prevents heterosexuals from their otherwise recognized liberties in the pursuit of happiness, what possible other interest does the government have in prohibiting this? Ought your policy goal to extend to the marriage of old couples past child-rearing age? How about the infertile? Should existing marriages be dissolved after the children have become economically self-sufficient? Or might there be, just maybe, just possibly, a common good coequal with the rearing of children, one that governments have an interest in promoting if governments value anything beyond their own survival? This may or may not be a quantifiable goal--I let others take me to task on that--but I think it is pretty damn compelling neverthess. I just can't buy the argument that gov't recognition of marriage cannot possibly promote both healthy partnerhips and effective baby production lines at the same time, and that both things aren't worthy of falling under the same title. But that's just me.
(Note: We haven't even touched the religious issue, which is, I think, far thornier for a lot of people than simple policy goals, unless you're talking about the benefits of conflating religion and economic policy goals.)
(Note 2: Other untouched areas: homosexual couples adopting children; the "normalization" of homosexuality; how religious liberty could be affected by homosexual marriage; since legal divorce has effectively gutted the permanency goal of marriage, should "marriage" fall strictly under the purview of religious institutions and "domestic partnerships" strictly under the purview of gov'ts? Probably.)
