Really, this is more like traditional marriage's last stand than a battle to protect traditional marriage. If it helps, think of this as rather like the fight of the copyright interests to get the DMCA passed. It is problematic and annoying, but it is no more possible to hold back the disruptive tide than when Canute gave his courtiers the object lesson with the real thing 1000 years ago.
I've tried a couple of times to write this, and it invariably goes on for many, many pages trying to explain what traditional marriage has meant from a biological, economic and social stand point over the last 100K years or so of human history. So i will give up on that and just start asserting stuff and possibly will explain in comments.
"Traditional" marriage, meaning 1 man and 1 woman in some form of legally recognized bond conferring certain rights and creating certain responsibilities, is one of the most common social patterns in the world. The only notable exception is that many systems technically allow one man to have more than one wife. In practice, however, polygamous societies usually limit polygamy to the wealthiest, most socially successful males or take place where there is a large number of females relative to males.
There are a number of reasons proposed for the ubiquity and stability of this pattern. Since this essay gets too long every time I try to describe them and why they fail to provide a complete picture, I will limit myself to the observation that, like any stable and self-perpetuating legal system, it develops a class of incumbents that benefit from the system, or at least fear that any instability will work to their detriment.
For about the last 150 years, traditional marriage has been declining steadily. The first assault came with technologies that permitted even limited control of fertility in the form of birth control. The second -- and related -- issue was the access of women to significantly greater economic and educational opportunities. The third, and really the most devastating, was the advent of no-fault divorce. All three of these combined to substantively alter the nature of the institution of marriage.
It is often tempting to believe these were universally favorable to the class of women incumbents. They were not, as evidenced in part by the fact that a substantial number of women opposed each innovation -- a phenomenon completely at odds with other aspects of the civil roghts movement. There was never an organized pro-Jim Crow lobby among African Americans. There were, and still are, a substantial number of women who oppose all these innovations. These were women who perceived -- often correctly -- that the widespread acceptance of these new technologies or social innovations would fundamentally alter the nature of marriage and the social order to their detriment.
The most visible example of this was the advent of no-fault divorce. The class of people that experienced the most immediate detrimental effects of no-fault divorce were middle aged women, especially those with under age children. In economic terms, these women suffered a complete disruption of their previous business model for survival without any economic or social safety net. One may argue that this was necessary to achieve greater social good for all, and I might agree. One may also argue that the legal change merely reflected the social reality, and the attempt to persist in the previous legal scheme was detrimental to the rule of law and to the institution of law as a whole, and again I would agree. But just as I do not expect the music industry to appreciate why arguments against more restrictive controls on copying are bad for everyone -- even the music industry -- I do not expect the class of married incumbents to recognize the a complete reversal of the social order is in their best interest when it makes them economically and socially worse off in the short term.
So how does this relate to same sex marriage? It is easy to see why birth control (and, later, artificial insemination and other forms of fertility treatment) undermined one critical aspect of "traditional" marriage by making it possible for sex partners to decide (within limits) whether or not to have children. It is easy to see why giving women a greater scope of access to economic and educational resources altered the previous calculus and undermined another critical aspect in the quid-pro-quo of traditional marriage. Finally, it is easy to see how no-fault divorce made the dissolution of traditional marriage easier and again altered its social calculus. But same sex marriage does none of these things. It has, on the surface, no impact on traditional marriage at all. Few seriously expect that allowing same sex marriage is going to "give your spouse the gay." So what makes same sex marriage the last ditch defense of the traditional marriage incumbents?
The answer lies in what same sex marriage symbolizes. Critically same sex marriage completes the transformation of marriage over the last 150 years from an institution based on a social and economic quid-pro-quo to an institution wholly defined by individuals. Same sex marriage cannot be justified on the basis of any biological fact, economic rationale, or political philosophy other than the intrinsic belief that marriage is a formalization of a relationship between two people in a manner utterly defined by the individuals.
And that is truly revolutionary. It flies in the face of every justification of marriage traditionally offered -- including modern appeals to evolution and "male sexual jealousy." As with many revolutionary social ideas, it is really just a recognition of what is already happening. Scholars may debate when the fundamental nature of marriage changed from an accepted part of the social order that imposed a particular set of written and unwritten rights, privileges and responsibilities -- but the recognition by the State of same sex marriage places the ultimate imprimatur on this conclusion. Even religiously defined marriage is, hereafter, merely the consensual fiction of all fellow believers rather than a manifestation of the social order. To say that Mormons or others "don't need" to recognize same sex marriage is simply to reenforce for them that while they may create an artificial world that excludes everyone else, they cannot dictate their business model to the world. For an incumbent, the inability to control the business model is always unsettling.
It should be noted that the disruptive change does not impact all "marriage incumbents." It only impacts those who view perpetuating the existing structure around marriage as to their advantage. As the bulk of the population has already made the transition to marriage as self-defined rather than marriage as defined structure, it has little impact.
At the same time, it should be noted that the change is disruptive because it induces change in society as a whole. Whether a particular individual used or did not use birth control did not impact how birth control changed attitudes about sex and clearly had enormous impact on the choices made by individuals. The expansion of educational and economic opportunities for women had impact on stay-at-home Moms precisely because it was now a choice and not a societal norm. Married couples act in the shadow of divorce at will. Again, you can chose to limit your own exposure, but the incumbent loses the market rules that it understands.
And that is something that incumbents fight like Hell against, even when they don't understand it. I just got through a huge fight with incumbents in the broadcast bands over access to spectrum. Do you think Dolly Parton has the least understanding about the potential for intereference with wireless microphones? Yet she ponied up like a good little incumbent because people she trusts in her incumbent world (Grand Old Oprey) told her it was bad. What makes anyone thinks that parishoners in churches are going to behave differently?
Again, don't expect the changes to be automatically better or worse, or for the benfits to be evenly distributed. Disruptive change is rarely tidy. But stop minimizing the idea it is revolutionary or engaging in foolish slogans such as "my marriage doesn't need protecting" -- unless they are genuinely intended as misinformation by the vanguard party. Because this is a revolution. Or, more acurately, this is the end of the revolution that began long ago. Time to embrace it and move on.