Check out H. J. Res 93 (in PDF format), Proposing an amendment to the Constitution of the United States relating to marriage. The text of the amendment goes a little something like this:
"SECTION 1. Marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution or the constitution of any State, nor state or federal law, shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups."
As the saying goes, "that's all she wrote." Nice, huh? If you get a chance, contact the Congressional type person of your choice and politely ask them to vote no on this one. The Feminist Majority Foundation has a more detailed article available, which also includes a pre-fab e-mail you can shoot off to your representative.