In other words......there is no such thing as an "equal right" across the board.
In a general sense, correct. There's not. I don't know why you, or anyone else, would have that foolish idea. The government discriminates against a **** ton of people in this country. People under a certain age, people of a certain gender, people of a certain race, people who haven't served in the military, people who have committed crimes, people who have an education, people who have worked for the federal government, people who interact with foreign entities, people who have certain citizenship status, etc.
In a more specific sense, incorrect. We discriminate a **** ton. HOWEVER, that discrimination must reach constitutional muster to be allowed. And there is Equal Protection under the law, in so far that EVERYONE is granted the protection of any discrimination that doesn't meet the constitutional requirements. Essentially...people have "equal rights" in the sense that their rights can only be restricted based on a standard that is applied evenly across the board.
In the case of Same Sex Marriage, as I stated, I can make a compelling argument (one that has passed in part through some courts) that such a level of necessity on the part of the Government is not reached. (and there's a less strong, but still somewhat compelling, argument in terms of Gay Marriage)
You would need to do similar to say Polygamy should be allowable.
Those seeking same sex marriage shouldn't get it becuase "Everyone should be totally and absolutely equal across the board". Those seeking same sex marriage should get it because their rights are being inequally applied by the standard that others are given. I've seen no compelling argument, at this point, for Polygamy that qualifies in that same fashion. If you have one, please present it.