"there is no justification for denying law-abiding, taxpaying Gay couples the same legal benefits and protections that Straight couples have always taken for granted."
It's the same justification for why the government discriminates between drivers with licences and drivers without licences - one has not met the requirements to be considered legitimate. If homosexuals want the benefits of marriage, they are free to marry according to the same standards as all other citizens. They do not have the right to redefine marriage to fit their own whims. Such an act would undermine the justice system, which is built on discriminating between behaviors.