Marriage is the most important institution of a happy society. It is the basis of the nuclear family, which allows millions of people—children and adults—to live stable, meaningful lives. Married people are less lonely throughout their lives than unmarried people. A marriage is the basis for raising a son or daughter in a stable environment. And it allows us to domesticate sex and put it to good purpose.
So it is right that the state should encourage people to marry: not necessarily to give any economic benefit to citizens, but to insure their happiness. Citizens should receive heavy tax breaks if they get married. Raising children in a marriage should be subsidized even more than it is now.
But I shouldn't give the impression that the state should treat marriage like it would any other contract. Given that there is a strong public interest not only in enforcing but also in encouraging it, the law—as it should—has always distinguished marriage from ordinary contracts. It is much harder to escape, for instance: not until 2010 did every state permit couples to consensually divorce. Moreover, it is a contract in which the state is an interested party and which it can regulate at will.
Accordingly, we should avoid calling marriage a "civil union", as if we should leave it up to the partners involved to imbue their contract with whatever meaning they want. To the contrary, marriage has a specific meaning that the state should impose on everyone who gets married. It is a lifelong, exclusive relationship between loving people; who commit to staying together, supporting each other financially, and to starting a family if they want to and can. This union holds up society and makes its participants happier.
In sum, marriage is the institution of our society most worth upholding, and whose meaning it is the most important to preserve.
And that's why I support gay marriage.