There's a point that Scott Nolte distilled from my earlier posts that I neglected to mention: this is not an issue that can be left to the states. The federal government has an interest in the definition of marriage, certainly in the tax laws, but possibly in other areas, too.
This is a much more complex subject than many assume. We've already seen that letting a single state's judiciary decide the issue affects the entire country, and it appears that the majority of Americans oppose same sex marriage. But once again, our federation makes for a sticky situation.
I strongly favor letting states handle as much as possible, but the definition of marriage is truly a national issue. I'm not sure how the federal government goes about defining marriage, but it's work that needs to be done.