Those in favor of gay marriage believe that those of us who hold the traditional view are "on the wrong side of history." Maybe so, but history's a funny thing. When we point out that no major society -- ever -- has given full recognition to homosexual marriages (however much they may have turned a blind eye to gay sexual relationships), we are told, so what? And when we invoke the slippery slope argument, that as soon as we legalize gay marriage, we'll be asked to legalize something else, they say we're just setting up straw men. But here's the thing. Gay marriage is entirely new -- but there are a lot of marriage arrangements that we discarded in favor of the (more or less) Christian model of marriage, which were once perfectly legal and could be again.
1. Polygamy. Having more than one wife is perfectly legal in many parts of the world. It goes on in this country, even if it's illegal everywhere. The US Congress refused to make Utah a State so long as the LDS Church practiced polygamy, and the newly-revised eternal truth showed up right on schedule. I doubt that that would be the big deal now that it was then. Those advocating the incorporation of parts of Sharia law into Western law codes are already figuring out how to accommodate plural marriage. Of course, being the open-minded types that we "progressive" people are, I suppose that polygamy (plural wives) is too old-fashioned. We will eventually be asked (I know those asking for it now) to affirm polyandry (plural husbands) and polyamory (any number of persons of either sex in a marriage).
2. Concubinage. Many societies have made possible having more than one legal relationship at a time by declaring one to be "marriage" and the other(s) something else. In the early 19th Century -- even after incorporation into the USA -- the Creoles of New Orleans practiced a form of concubinage. Wealthy white men married white wives to provide heirs, and also contracted stable relationships (including legally enforceable transfers of property, etc.) with mixed-race women called placees. During the social season, these men would divide their time between white balls and colored balls, hosting both their women and spending time with both their families. It was called "the custom of the country."
3. Child marriage. The age of marriage has fluctuated a bit over our society's legal history. Once arranged marriages fell out of fashion, the prospective partners had to be of age to consent to the relationship. This has usually meant that you had to be at least a teenager to get married legally, though some have gotten married very young. I have a friend whose grandmother was married on her fourteenth birthday. Jerry Lee Lewis married his cousin when she was thirteen, though that became a public scandal. Nor are we just talking about two very young people getting married. Lots of very young women married fairly old men in the late 19th Century when the first Civil War veterans' pensions were offered. Suddenly, the old guys had a lot to offer the sweet young things. My great-grandfather, Civil War veteran Zechariah T. Pulliam, was older than his father-in-law. (By all accounts, my great-grandparents were very happy together.) There's nothing magic about the age of consent. Once you decide that we can redefine marriage any way we want, it's all about who wants an outcome the most and how many votes they can scrape up. And a lot of the bold, crisp lines we've drawn to protect the young become very faint and fuzzy.
Tradition doesn't just mean "old." It means also, what has been shown to work. But other things used to work, too; we just didn't find that they fit into the kind of society we were trying to be. We didn't magic our "traditional" view of marriage out of nothing, but we dropped some things along the way, emphasized this a bit more than that. We evolved, to borrow a phrase from somebody. But remember, if we're going to ignore the wisdom of tradition because we think it doesn't matter, then there will be no argument that will hold water when somebody presents one of these old arrangements as something "new" and greatly desired by VOTERS.