A lot of my thinking on this grows out of the point that the US, and the West in general, is becoming less homogenous all the time. It's not just immigration. With the breakdown of Christianity as the underlying metaphysics of Western society all sorts of other ideologies are coming into play. Secularism is growing but still undefined in many ways. What is the secularist view on marriage? I don't think there is one. Eastern religions are gaining ground, Buddhism doesn't offer any rules on marriage. It just says if you are married you should be good about it. How vague is that?
A second line of thought for me has been, it is necessary for all 50 States in the US to agree? They don't now on all sorts of things. In New Jersey you cannot have self service gas stations. They are common in New York. Marraiges aren't gas stations, but where is the line?