However, history has shown us that in cases of deep conviction, politicians will break with status-quo rhetoric and take risks in support of issues in which they truly believe. Jon Corzine did it with New Jersey's budget negotiations in 2006; John Kennedy and Lyndon Johnson did it with civil rights; Howard Dean did it with the Iraq war, etc. Given this, the propensity of current political heavy-hitters to dodge the looming issue of equal marriage rights must owe to more than a simple desire for electoral support.
More than anything else, I believe that the institutionalization of "Don't Ask, Don't Tell" policies concerning homosexuality in the military and in a variety of other private and public professional sectors in America. These guidelines do more than simply keep LGBT individuals in the closet: rather, they prevent the friends and colleagues of these individuals from having to consider homosexuality as anything other than an abstraction. When sexuality remains an abstract construct rather than a concrete component of everyday life, people of "mainstream" sexual orientation can easily view homosexuality as something abnormal or strange.
To put it very simply, the institutionalization of gag rules concerning sexuality prevents a large portion of the American populace from ever seeing just how very normal and unremarkable homosexuality is. Individuals who form attractions toward people of the same gender aren't fundamentally different from the rest of humanity in other ways; they read the same books, wear the same clothes, laugh at the same jokes and make the same mistakes in life and in love. But as long as our government treats homosexuality as a sordid secret, a great many members of our society will not realize these things, because they will never know that some of their closest and dearest friends are attracted to people of their own gender.
For people who stick with the status quo, it seems easy to say, "I believe marriage is between a man and a woman" when you believe that your words have no consequences for anyone about whom you care. It becomes more difficult to endorse this belief when you must do so knowing that you are closing doors for the future of a friend or family member. Even Dick Cheney choked up a little during the 2004 Vice Presidential debate when he mentioned his daughter in the context of marriage equality legislation, and gave a succinct but emotionally charged explanation of his knowledge that to get elected, he needed to stand by President Bush on this issue despite his own personal experience as the father of a lesbian-identified woman.
I do not think this argument sufficient on its own to explain why politicians today continue to evade the issue of marriage equality, but even standing alone, it holds quite a bit of water. Most prominent federal politicians come from older generations, and consequently are likely to work with and know people who do take a quieter approach to discussion of sexuality. Whereas many younger local and municipal politicians have announced their own identification as LGBT and/or ardently endorsed marriage equality, the higher average age of federal politicians seems a likely factor in stalling progress on this issue at the national level. Compare the preponderance of openly homosexual or allied municipal and county politicians to the relatively low number of openly homosexual or allied state and federal ones.
One could extrapolate from this logic to argue that in due time, the problem of reluctance to lend full support to the marriage equality movement will resolve itself as younger, more progressive politicians come of age. However, the issue truly at hand here is not merely the treatment of marriage equality by politicians, but rather the willingness of our society as a whole to open the door on the closet in which we keep our LGBT peers. Until we do so, gag rules will remain functional in a de facto sense even if their de jure counterparts get removed from the personnel policies of our professional organizations.