Traditional marriage isn't exactly the "one man, one woman" story you've always heard. Politicians may claim that the definition of marriage hasn't changed in thousands of years, but it most definitely has. Marriage has been through a lot of changes over the millennia -- most good, some bad.
For example, in Mesopotamia of 4,000 years ago, marriage was similar to slavery. The laws even included the rules for getting a refund if you weren't satisfied with your wife.
Romans found public displays of affection offensive, and one senator had to resign after he was seen kissing his wife in public. Early church officials discouraged people from marrying altogether, since it distracted them from praying. And a thousand years ago, girls were married when their ages were still in the single-digits.
For a long time, marriage was a lousy deal for women. Bernard of Siena told his male parishioners that they should show their wives as much mercy as they would a pig, and even Martin Luther said that he hit his wife for being "saucy."
It's only relatively recently that it's been common to marry for love. About a hundred years ago, conservatives warned that "love marriage" would destroy the institution. And it's within the last few decades that America overturned laws against interracial marriage, and giving husbands extra rights over wives.
Those interracial marriage bans, by the way, were justified as a form of "religious freedom," language that seems awfully similar to the justifications given today for banning marriage for same-sex couples.
Throughout all those changes, one things has stayed constant: marriage has always been an agreement between people. And as societies have changed for the better, they've updated marriage to change with them.