It's the basis for one of the arguments in the ruling against CA Prop 8: maintaining separate but equal institutions under different terms inherently means one is second class. Obviously "marriage" has more prestige than "domestic partnership" since the second is literally defined, in law, in terms of the first.
It was the camel's nose under the tent which brought us where we are now.
Yup. Folks have been conditioned to think that marriage is defined by the state, that a piece of paper from the gov’t determines if one is married or not. So they easily accept any impossibility the state puts forth as “marriage”, after all it’s just another lousy gubberment contract.
Pope Leo XIII saw this coming over a hundred years ago.
Freegards
Marriage predates government and religion. Many of the privileges of marriage stem not from government, but from voluntary association. Members of a society may be far more keen to dispense charity to a married couple than to single individuals; part of the reason they are willing to do so is that they believe marriage means something.