Here's the text of the marriage amendment that Bush supports:
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.
The media is reporting that as relatively harmless, since it doesn't seem to explicitly rule out other forms like civil unions.
In fact, it does. Basically, it means that if a state passes a law that says marriage-like rights shall be conferred on gay couples, then the Amendment says, "Oh no you don't." It basically makes any civil unions law unenforceable. Anyone can challenge a civil union law and win. Any employer or state can refuse to give gay couples the rights that the civil union law guarantees them, and they're protected. It not only rules out equal rights for gay couples, but removes the teeth from all existing laws like Vermont's.