We decide as a country what marriage means. The definition was not handed down to us carved in diamond on a tablet. It was written by people just like you and me, based on the goals of having a legal contract called "marriage".
But we need to keep in perspective that Outsiders is only saying we should have a different word for it, but he supports civil unions. Really the difference is pretty small there, just a term. Some of us think that having a second term for gay marriages is just an avenue to make them unequal (like they are now.) Outsiders thinks you could make sure that it stays equal. Maybe he's right, but either way we're all arguing for legal, if not lingual, equality, and that's what's really important here. Besides, with civil unions, gay couples would just call themselves married anyway.