I frankly believe that stance is a false facade for many politicians who give lip service to opposing homosexual marriage when in reality, they actually support it.
In Obama's case, I believe he's blown his cover on the issue in a recent letter to the Alice B. Toklas LGBT Democratic Club, where Obama wrote:
As the Democratic nominee for President, I am proud to join with and support the LGBT community in an effort to set our nation on a course that recognizes LGBT Americans with full equality under the law. That is why I support extending fully equal rights and benefits to same sex couples under both state and federal law. That is why I support repealing the Defense of Marriage Act and the "Don't Ask Don't Tell" policy, and the passage of laws to protect LGBT Americans from hate crimes and employment discrimination. And that is why I oppose the divisive and discriminatory efforts to amend the California Constitution, and similar efforts to amend the U.S. Constitution or those of other states.In the course of his letter he supports repealing the Defense of Marriage Act which defines marriage for purposes of federal law as the legal union of one man and one woman and protects the rights of states not to be forced to recognize same sex marriages from other states. He supports extending "fully equal rights and benefits to same sex couples". And he opposes the California Constitutional Marriage Amendment which is the only thing preventing that state from being forced by the California Supreme Court to recognize homosexual marriage.
Given these statements I don't see how Obama can with a straight face say he doesn't support homosexual marriage. His only recourse would be reverting to the postmodern, linguistic gymnastics of saying, "Personally, in my heart of hearts I think marriage is between one man and one woman but I wouldn't want to impose my personal beliefs on others." Of course, do so would draw into question his personal integrity and character.
For purposes of truth in advertising Obama is, I believe, de facto in favor of homosexual marriage and more than willing to see it imposed, by judicial fiat if need be, on every state and thus every citizen in our nation.