Jack Balkin, legal scholar proposes amending the FMA to read properly. I agree with him. If it should be done, it should be done right. Damn lawyers! What they can do with a single sentence - they obfuscate purposefully.
If the FMA had been designed to do what its proponents claim it will do, it should have been drafted as follows: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 be conferred upon unmarried couples or groups.
Section 2. Nothing in the first section of this Article shall be construed to prevent either Congress or the legislatures of the several states from providing any other benefits, rights, or privileges, or combinations thereof, to unmarried couples or groups.
Thus, Congress and state legislatures may provide all of the incidents of marital status except marital status itself. As you can see, such an amendment is not particularly difficult to draft. The fact that there is a gap between what the text says and what the Alliance for Marriage says the text will do suggests to me that they are not being entirely forthcoming about the reasons for the Amendment.