Now, if you want to consider "threats" to marriage, consider Britney Spears and the one-day wonder quickie wedding in Las Vegas. Or consider serial marriages and divorces by any number of Hollywood types.
Gay marriage does not lead down the "slippery slope" to bad behavior (see Argument 9). Gay marriage does not change the values of marriage: faithful, committed, monogamy for life. Some of the pro-8 forces point at excesses from Gay Pride parades. They are no more representative of gay marriage than the college-student hook-up culture is to straight marriage.
But let's also consider what the traditions of marriage really are, shall we? The Bible is full of examples of polygamy, concubinage, forced marriage and child marriage. (Indeed, I am sure that I am not the only one amused at the delicious irony of the Mormons, with polygamy still "on the books", claiming they are the protectors of "traditional marriage").
Women for many years were merely representatives of property, and indeed WERE the property of their husbands. That was pretty "traditional" too, and still is in many cultures.
In fact, our notion of marriage based on the love and free choice of two people, is extremely modern and not the least "traditional". Marriage has already evolved along with our society and culture.
From the LA Times:
Marriage is unique because of the high social expectations that go with it. Chief among those expectations is that spouses will do whatever is necessary to care for each other -- which is valuable, because census data show that almost a third of California's gay couples have only one wage-earner, and almost a fifth have at least one disabled partner (about the same, by the way, as for straight married couples). By supporting and reinforcing the care-giving commitment, each marriage, gay no less than straight, creates social capital for the whole community.