Getting the government out of marriage strikes me as the proper libertarian position.
Over the millennia, promoting, encouraging, and rewarding the traditional marriage of one man and one woman have proven to be the best way to maintain the society. All of a sudden this is no good?
"If this allows some homosexuals to gain the benefits of domestic partnership, I fail to see the harm in that."
What you fail to see is that recognizing "domestic partners" or even "homosexual marriage" is not the goal. The goal is to add legitimacy to that lifestyle choice. It's saying that we, as a society, make no distinction between the two. And that's not correct.
I think promoting traditional marriage is great! We should all do it!
My point is that government involvement will, in the end, be destructive to traditional marriage. Our government is incapable of protecting an institution like marriage by saying it should be open to some and not to others. It will default to the inclusive position, time and time again.
So if one supports an inclusive definition of marriage, getting the government as intertwined as possible is the one sure way guarantee that the definition is expanded. Once the government hangs a whole lot of legal rights, responsibilities and goodies onto marriage, there is no way to restrict it to certain people.
The FMA will never happen. It is a losing rear-guard action based on the notion that government activism can have any result other than the opposite of what is intended.
Since government will inevitably destroy the institution of marriage, I propose keeping government and marriage as far as possible away from one another.