This link will take you to an informative and amusing set of YouTube videos by John Corvino attempting to refute arguments against same-sex marriage. Corvino states his responses to these arguments in a very clear, easy-to-understand, and effective way.
Although I am against legal recognition of same-sex marriages, I am actually very sympathetic to many of Corvino's arguments. I think this series points out very well how weak secular (that is, Agnostic) arguments against same-sex marriage really are. I cannot think of any really good reason not to legally recognize same-sex marriage, assuming an Agnostic worldview. The lesson for us Christians is that we should stop trying to play this game on Agnostic terms and instead make our arguments on the basis of the real reasons we hold our position--our Christian beliefs and values. "But then our arguments will be rejected as a violation of the separation of church and state!" Yes, that is probably true. At that point, we have to challenge the principle of the separation of church and state, which is the endorsement of Agnosticism as the official worldview of the nation. It may be harder to carry on the conversation in this way, but we will be honestly dealing with the real issues instead of trying to win by substance-less rhetoric. We will be fighting not only for one practical issue, but holistically for an entire outlook on political and social ethics grounded in Christian beliefs and values. This will make the conversation far more worth having; and if we win, the win will be far greater and on a much firmer foundation.
(By the way, I heard Corvino speak at Utah Valley University yesterday, and I was struck by his dialogue-minded spirit. Some people are a lot harder to talk to than others. My impression is that Corvino is someone with whom one can have a real conversation about controversial issues. We all should cultivate this kind of substantial and civil dialogue whenever possible.)
UPDATE 11/18/2014: See here for an examination of the Proposition 8 court case in CA where some of the themes mentioned above are examined at greater depth.