One other point I think is important:
I hear a lot of people say, "the government needs to get out of the marriage business." While I understand the sentiment, as an argument this fails on a few levels. First, realistically, what are the odds that the government is going to just "get out of the marriage business"? Do the people who say that really think it's going to happen? My guess is it's highly unlikely that will ever happen (at least in our lifetimes), so as a point it's mainly a cop-out because it's never going to happen.
Second, rhetorically, this seems like a cop-out because it's always mentioned in relation to the "gay marriage" question. No one asked if you think the government should, generally speaking, recognize marriage. Do you agree with legal gay marriage, yes or no? My guess is many people who make this argument don't want to seem like bigoted homophobes by truthfully saying they don't believe in it, so they go for the scorched-earth approach that offends no one.
Third, and perhaps most important, the government isn't in the "marriage business" anyway. The problem is a lot of people who don't understand the law (activists and bleeding-heart liberal douchebags on message boards) get to frame this issue, and they rationalize points that aren't realistic. There is no marriage entitlement program you're kissed into when you get a marriage license. "Marriage" in the legal sense exists as any number of legal stipulations in state and federal law. You can't generalize these stipulations as being "rights", "protections" (for those of you who try to link this social issue to some invisible constitutional tenet) or even "generally significant" to most couples -- even straight ones. For one, there are too many of them and no one has gone over each iteration of "marriage" to determine it's effects on a couple; two, it would relatively impossible to do that because individual couples vary greatly, not just from person to person, but year to year (there is no uniformity even among straight couples, except the legal part); and three, most of these iterations exist within a larger framework that matters more than just marital status. For example, a mere count of the times marriage is mentioned in Veteran's Affairs policy might be in the hundreds, but the larger point is those can only pertain to spouses of veterans. Ditto Social Security, Medicare, Medicaid, and tax policy. So if a list is compiled of all the instances of marriage in federal law, it's easy for someone to just conclude that there are hundreds, if not thousands, of rights and benefits gay people can't receive, but that's not the case.