I support marriage equality. Two consenting adults should have the right to marry if they so choose. Nothing about this has ever struck me as being particularly controversial, even though I have known quite a few people during the course of my life who seemed to have a problem with it. When asked if I support same-sex marriage, my response is usually something like, "Of course! Why wouldn't I?" I thought it might be interesting to see if I can articulate why.
I suppose the short version would be that I support marriage equality because I support equality itself. I see no reason why two people should be denied basic human rights simply because of their sex or sexual orientation. And yes, I admit that I find it puzzling how anyone could think otherwise.
I have heard all the same arguments against same-sex marriage that you have. Most are religiously-based, and none have been even mildly persuasive. Opposition to same-sex marriage is no more defensible than opposition to interracial marriage. Simply put, it is bigotry.
The good news for those of us interested in promoting equality is that the recent advances made by the LGBT community are exposing considerable bigotry to the light of day. Most people are disgusted by it when it is revealed to them, and this should make it more difficult to sustain. This is especially true when we see that the dire predictions many Christians make about the effects of same-sex marriage will not materialize.
As an atheist, I recognize the debt of gratitude I owe to the LGBT community. Their efforts have not only paved the way for us, but they have modeled so many effective strategies for advancing equality in the face of Christian extremism. Even as it saddens me that some LGBT persons will oppose atheist equality, I recognize how we have been aided by their progress.
H/T to Unreasonable Faith