I guess the real nut of what bothers me about government privileging marriage over other family structures is that it attempts to regulate sexual behavior. Only if you have this narrowly proscribed type of sexual relationship (or at least profess to) do you officially exist as a household. Anything else is invalid.
Stripped (rightly so, because they are also the things that made it oppressive, mostly for women) of all the things that made it meaningful as an institution for community stability -- the strict gender roles, the more or less compulsory children, the near-impossibility of divorce -- there's nothing left of marriage except the being in love part, the sex part. And that's what bothers me about the same-sex marriage campaign, that it is based on the sentiment that everyone should have the right to marry the person they fall in love with. Why? Maybe we should have the right to marry the person or persons we commit ourselves to nurture and support unconditionally forever. Maybe. But the person we fall in love with? Why?