Marriage has a Darwinian ability to endure. It alters itself however necessary to continue to be relevant and useful in people's lives. It is a shape shifter. It is like cockroaches and alligators. Marriage will be here long after humans are gone.
People will always want intimacy with one chosen person and you cannot have intimacy without privacy, which is why couples draw circles of privacy around themselves. They demand that family, neighbors and the law respect their union, and that is why we have marriage.
As for the idea of creating such a thing as secular civil unions that would offer couples every legal advantage of marriage without using the language of marriage—as they now do in Europe—this makes perfect sense...or rather, it would if we Americans did not happen to hold the concept of "marriage" in such rapturous reverence. The problem is, in this country, a civil union will always be seen as a badge of second-class citizenship.
The Europeans do not share our innate cultural reverence for marriage, at least not the northern Europeans. The Portuguese still do. Here's the thing: the unit of reverence in Europe is the family, which is why a child born today of unmarried parents in Sweden has a better chance of growing up in a house with both of his parents than a child born to a married couple in America. Here we revere the couple, there they revere the family. This is also why homosexuals in Europe have no comprehension of why homosexuals in America are fighting for the right to marry: They are perfectly happy to simply have equal civil rights, without the language of marriage. But here in America, marriage still has a mystical, intangible power: It is a passport to adulthood and respectability and to a certain extent citizenship. Any relationship less than "married" is considered temporary and not worthy of honor.
It's unfortunate that there exists only one path in America to complete social legitimacy, and that is marriage. For instance, that it would be far easier for Americans to elect a black president or a female president than an unmarried president. That would truly feel like cause for suspicion. Which means—of course—there is a massive pressure to apply this particular shape to one's relationship. Which might explain why Americans marry more—and, sadly, divorce more—than anyone else in the industrialized world. So the downside is that there is a rush to the altar—couples want to earn that badge of instant respect—when they perhaps are unready, or not mature enough, to actually take on that commitment.
In the ancient world, marriage was a tribal bond—a means of legitimizing heirs and building family dynasties. In the medieval world, marriage was an economic bond—a means of safely passing wealth from one generation to the next. During the height of the Catholic church's power, marriage was a religious bond—a lifelong, unbreakable contract to God, sealed by a priest. During the Industrial Revolution, with the rise of prosperity across the Western world, marriage finally gained the luxury of becoming a bond of love, an expression of individualistic choice.
Today, marriage is a curious amalgam of all those things. Modern marriage is first and foremost a romantic and private union, but the tax laws and inheritance laws and religious implications that still surround this institution indicate that marriage has evolved without casting away its earlier purposes or assumptions. It's like we just keep building on this thing, piling new advancements on the old model.
Modern marriage as a car strangely fashioned out of an old abandoned horse carriage, built upon the framework of a mule cart. All the original engineering is still there, underneath it all.
Whether we are aware of it or not, we carry into our modern marriages the expectations and social memory of thousands of years of history, as well as our own set of newfangled tools that we use to tinker daily with the old machine. We alter and customize the thing every century, every generation, every day—both in the courts and in our own homes. And marriage accepts our modifications gracefully. Marriage adapts, evolves and somehow keeps chugging along.