For a number of reasons, firstly you are assuming that marriage is 'merely' a 'right' granted and defined by the state whereas that has not been established. Any 'right' granted by the state can be altered or removed by the state and is not an intrinsic basic human right. This is arguable of course depending on ones view of absolutes vs moral relativity so we can move on rather than get bogged down on that point because it is a matter for the individual. There are other philosophical reasons for not abrogating that kind of power to the state. One that has come up often is that if 'marriage' is to be defined by the state based on the present Zeitgeist then is NO argument you can use to justify SSM that cannot be used to justify any type of union.
Secondly: There is no tangible benefit conveyed to SSC's through SSM that they cannot (or do not)already have, in terms of true equality (using your examples of women and Aboriginals) those groups could not go and vote somewhere else. Things like voting are 'rights' legitimately conveyed by the state whereas marriage may not be. Marriage (the union of man to woman/women) seems to have been a global institution pre-dating both state and Church and therefore, logically cannot be a construct of either. They were denied totally from those things (voting etc) with no avenue of recourse. With SSC's if one Church refuses to marry them they can simply go to the Church/baker/florist next door (of course a lot of them simply prefer to sue or harass people for some reason....which leads me to question their true motives.) In short SSC's already have equality despite what the 'yes' mob claims about obscure things like visitation rights etc. That they might not 'feel' equal is a job for the psychologists and not the government.
That is a couple of reasons why this is not about equality. (Could probably think up more, but let's face it, what's the point?)
