It depends on what you mean in the question. I support gay marriage, but it has nothing to do with rights or equality (or overpopulation for that matter). I also wouldn't support polygamy, and absolutely not atrocities like incestuous marriages or group marriages.
To be fair ... Im not sure when filling out a particular government form for tax reasons became a protected right for every person out there. You can certainly declare to be whatever it is you think you are, thats fine, just dont fudge up any paperwork or accounting on our part.
All it will cause is for us to have to take marriage out of it as a factor and find other ways to benefit only those groups again. There was a reason behind providing those benefits to a man and woman intending to bear children. Theyll get brought back around and focused on them again one way or another.
@varrack there's good reason to oppose polygamy and incest. It's not a slippery slope. Your religious feelings are not good reasons to force your personal laws (which you can happily follow) on everyone else.