It is my obligation, not only as an elected official in a pluralistic society but also as a Christian, to remain open to the possibility that my unwillingness to support gay marriage is misguided, just as I cannot claim infallibility in my support of abortion rights. I must admit that I may have been infected with society's prejudices and predilections and attributed them to God; that Jesus' call to love one another might demand a different conclusion; and that in years hence I might be seen on the wrong side of history.
It reveals a man with a healthy dose of skepticism about his own beliefs but perhaps lacking the courage to take the step he knows he should. Is that better or worse than someone who is sure gay marriage is wrong and always will be?
Right now I'm going with better, since he at least has the courage to ask the question. I'd prefer a president who makes mistakes with the understanding that he is fallible to one who makes mistakes with 100% certainty. Presidents can't publicly precede the people to the right conclusion on every issue, but Obama has at least done so on one major issue: the Iraq war. If he merely rides the public wave of support for gay rights as it continues to grow, it's at least better than most of his rivals can be trusted to do.
Obama does, for the record, support laws against discrimination and for civil unions. He must also have an innate sense that the public can be wrong about questions of civil rights and marriage -- his black father and white mother were married in 1960, when miscegenation was illegal in half the states in America.