[Gelato]

Of course non-fertile couples can get married. That doesn't contradict the point that in principle, marriage between a man and a woman results in children, and that society and the government has an interest in protecting that institution. It's the principle, not the exception, that makes the rule.

[Mathlete]

Why does society get involved? ... It's because, throughout human history, there has been an acknowledgment that, well, marriage is about family; it's about having kids.

Actually, it's about increasing our population numbers as (1) to amplify gestalt, (2) to defer invasion, and (3) since evolutionary "success" is generally defined synonymously with fecundity, by sheer numbers alone. In every case, it's really about increasing the number of friendly citizens. These seems to be the strategy of all nations that subsidize marriage. As Richard Dawkins so brilliantly conveys, it isn't all about DNA. There's the culture (meme) that evolves as well. In other words, there is no such thing as the "American" race.

Reproduction is only one method for achieving these goals. Immigration and Hegemony are others. More important than these two is to increase longevity, as in reducing the number of casualties on the battlefield (by technological superiority or tactics), as well as at home (by increasing the lifespan of humans). It is this third reason that best logically supports "gay marriage" with the "still in infancy" argument that married couples tend (more often than not) to have less sexual relationships ('one' being the ideal) than non-married couples. Since AIDS spread by promiscuity, and AIDS kills, AIDS reduces our population count, and wastes our resources both labor and material.