We could use the same description to define something way more simple: a heterosexual sexual act. That was nature's design, anyway, at least if we look at the animal world, where no special ceremony is necessary. Human societies took that act and confined it to one man, one woman, at least for the common people. The king always got to have multiple wives and concubines. Even Biblical figures did this. But I'm sure you're aware of that.
Moreover, almost every union has the potential for producing children.
You can forget my potential marriage with my lady. See my comments above as to why it "ain't gonna happen". Given that, would you deny us the benefits that marriage would supply to us, even though we have zero possibility of producing children to protect with the societal institution of marriage?
Even if you don't believe in the religious aspect, marriage between a man and a woman is socially, potentially, and ontologically different from a necessarily barren homosexual relationship.
You mentioned adoption earlier in your response, homosexual couples can adopt children in the vast majority of states. They certainly earned their stripes in this endeavor when they took on AIDS babies that nobody else wanted. And with a pair of lesbians, all they need is a donor, and both of them can have children if that is what is desired. There's not a single thing that our society can do to stop them, if there were, we'd use that to prevent welfare mothers.
I know that socially speaking, it has been one man, one woman in most Western societies for a very long period of time. I also know that a mere two hundred or so years ago, the men that I deeply admire as the Founding Fathers regarded full political freedom as something that could only be extended to white, property-owning males over the age of twenty-one. Clearly, we've seen a social evolution from beyond that point.
Perhaps the institution of civil marriage, which is the only religious institution co-opted by our legal system, would evolve as well. That's what happens when things that should have stayed within religion become part of the law.
But that's not the point. The point is that despite individual accidents of health, fertility, or whatever, a marriage between a man and a woman, as a general rule, results in babies as a natural consequence. You can always find individual exceptions, but a homosexual relationship can never be fruitful. Ever.
Using the lower animals as a model for human behavior is reductionist and ridiculous. We are not animals, more is expected of us. Animals kill one another freely and devour their children, the males will kill the female's preexisting offspring, etc.
The whole in vitro thing is a time bomb waiting to go off. People freak out because the Catholic Church opposes it, but it can be disastrous, especially with homosexual couples. I work in the justice system, and you would be surprised how many male homosexual couples molest adopted male children . . . and how many lesbian couples physically abuse and even murder male children. I personally know a lesbian couple with an AI male child, and that is the most messed up kid I know. The fact that some well-intentioned couples have adopted AIDs babies is a star in their crown, but they are not all that common. Far too many of the babies are political trophies. And to use the rare exception of a well-intentioned adoption (which is a cure for a tragic problem, not the norm) as a justification to change the entire definition of marriage, is again reductionist.
And to compare the franchise with marriage is like comparing homosexuality with race. Apples and oranges. The franchise as a concept isn't anywhere near as old as marriage -- go back a thousand years and hardly anybody anywhere got a "vote" except some of the Scandinavian peoples, and there it was only the warriors who got to vote.
Marriage has been both a religious and a social construct for as long as there has been society, because otherwise kids don't know who their daddies are and daddies don't know who their babies are. The social purpose of marriage is to legitimize offspring and thus reassure the father that his genetic potential is actually being carried on.
Somebody at work said just yesterday about these sperm banks where donors are used over and over again, "We're going to have people marrying their sisters."