A few responses to Jonathan then I'll let this one go. (I'll also note my last name has one "o" in it.)
I do indeed recognize the state/private distinction and that distinction is at the heart of why I think, in the world of the second best of state-sanctioned marriage, same-sex couples should be included in the institution (and why, short of that, state universities and governments should extend the benefits they provide for their married employees to same-sex couples).
I guess it also bothers me to have this issue reduced to the power of the gay lobby group. Of course, there is a lobby group component to this issue, but characterizing it as primarily that obfuscates the real human beings and their real desire to partake in the institution of marriage that comprise those arguing for same-sex marriage rights. In this way, the movement for same-sex marriage shares elements with other civil rights movements of the past. Note, I'm not equating them; I'm simply suggesting they share some elements. Yes, there are material benefits involved, but I think most of the folks who really want this to happen, and who stood in line in Massachusetts to get a license, are much more concerned about being able to call themselves married and to be recognized as equal in the eyes of the law then they are with that second driver discount on the auto insurance.
As for the polygamy question... I won't rule out the possibility that polygamous marriage should be recognized by the state, but I think there's strong reasons to believe they are different from two-party marriages, and Jonathan Rauch's new book makes this case better than I can. If polygamy can reach the same status of tolerance or acceptance in civil society as have homosexuals and same-sex relationships, then let them make their case. But given some of the power issues involved, many of which doomed the institution historically, such relationships are less likely to survive.
Two last things. One, the slippery slope argument doesn't fly. It's certainly possible that extending marriage to same-sex couples would lead to further calls for legal intervention that libertarians wouldn't like. So what? If extending marriage is the right thing to do on libertarian grounds, then do it and deal with the other problems when they arise. Is the eventual emergence of forms of affirmative action that libertarians object to an argument for not ending slavery or not extending blacks the right to vote because that got the ball rolling?
Second, there's this paragraph:
And, you skirt the issue of monogamy entirely: The classic argument is that monogamy makes for social stability (no unhappy mate-less males or females, as in polygamous societies). Another argument is that it is better to have two parents raise children; this is supported by social science research.
What's to skirt? Is the claim that gays and lesbians are less likely to be monogamous than straights? If so, let's hold that thought until the comparison is fair - when gays and lesbians have real legal marriage to be bound to. Comparing the monogamy of married straights to unmarried same-sex couples proves nothing, other than perhaps that marriage would increase the level of monogamy among same-sex couples. And of course it's better ceteris paribus to have two parents raise children. The social science research also strongly suggests that the parents need not be of the opposite sex. One argument for same-sex marriage is that it creates more two-parent couples to raise children. (This doesn't even address when ceteris isn't paribus - better a stable one-parent family than a constantly conflictual marriage.)
I think it's possible for libertarians to simultaneously argue that the state should get out of the marriage business and that, given that it's in the business, the state should extend marriage rights to same-sex couples. I might think the state should dramatically reduce the level of taxation, but also believe that at the given level, taxes should be administered non-discriminatorily, even if that means some folks' taxes are higher.