I, however, am vey much in support of same-sex marriage.
"Traditional" anything is not necessarily always the best thing, just like "untraditional" is not necessarily always the worst. One could flip those, too, of course; "untraditional" is not always the best, just like "traditional" is not always the worst. I've noticed that we often use "traditional" or "untraditional" (or "nontraditional") to promote acceptance or disapproval, which presumes that either of these terms are wholly good or bad.
The problem is when one defines "traditional" marriage as the only way, simply because that's how one was raised, and uses those definitions of "traditional"/"nontraditional" to limit or deny others' rights. "Traditional" marriage is, I see, being used by more conservative, religious people who argue that marriage is Biblically-based, and is and should remain between one man and one woman for the purposes of the propagation of children.
There are problems with that, of course. Not everyone in a "traditional" marriage has children (for many reasons, some of which are of their own choosing, but many of which are not); some couples should not have children. "Traditional" marriage can be too narrowly defined, especially if gender roles and expectations are at the heart of the marriage. That can be just as damaging to everyone within a family, especially if one does not fit easily into those stereotypes, if one wants something different for oneself. There are women I know who delight at being stay-at-home mothers, whose husbands make the primary financial and family decisions, who are the head of the house. If that's the choice that's made within the family, there's nothing wrong with that. That is not the kind of marriage I want, though, and it would really chafe if those roles were expected of my husband and me.
I don't want a traditional marriage. I don't think I'm in a traditional marriage: I have more education than my husband; I work (and in a career that requires an advanced degree); I don't have children; I would not find satisfaction and fulfillment in being a housewife. It is not solely my husband's job to provide financially for our family. (And thank goodness I have the capacity to provide solely should something happen.)
There is absolutely nothing wrong with being as educated as one's husband, or one's husband being more educated; similarly, there's nothing wrong with choosing not to work, with raising children, with being a housewife; not every choice is the best one for each family or individual, and these choices are often made within the confines and in the best interest of individual families. However, to expect others to choose the same "traditions" that you do simply because of "tradition" is a very narrow view.
More damage is likely to occur if someone divorces multiple times, or practices polygamy. (That's another discussion right there, isn't it: Whether the Bible traditionally "allows" divorce or polygamy and whether those practices are still accepted.) It's ridiculous to limit the rights of two consenting adults who understand as well as anyone can before getting married what being married actually entails. Presuming that same-sex couples don't have the ability or maturity to enter into marriage because of their sexual orientation is fallacious and ignorant.
"Traditional" and "untraditional" are neither good nor bad; the harm comes when one tries to limit others' choices because of one's owns views. Not everyone should get married, regardless of sexual orientation. (In Catholicism, we consider marriage a calling, just like some are called to the single life, or to have children; they're not universal, unilateral callings.) "Traditional" or "untraditional" (or "nontraditional") should no longer be means to promote acceptance or disapproval.