There has been a surprising amount of gloom and doom about the FMA about these parts. The problem, I think, has to do with the framing of the issue.
The debate about the FMA does not need to be about "gay marriage"; it should be framed as a discussion about these core values:
Federalism; Equality; Liberty and Autonomy; Fairness; Separation of Church and State; and Constitutional Restraint. Opponents of the FMA should also emphasize that the FMA threatens both Civil Unions and the institution of Marriage itself.
Properly understood, the proposed Federal Marriage Amendment would tread on so many core principles . . .
. . .that it is unworthy of serious consideration for inclusion in our Constitution.
Here are some important values that are at stake:
Federalism - the FMA side is against traditional federalism: they want the Federal Government to do something it has never done before, namely regulate marriage. That is a core state function. Some states have community property; others don't. Some have different age limits from others. Some have different rules about cousin marriage from others. Conservatives who don't want Federal meddling in local and state affairs should be strongly opposed to this attempt to destroy state power and increase federal power.
Equality - equal protection is a core value, and every amendment to the constitution so far that has dealt with equal protection has extended it. The FMA is anti-equality, and it would be the first amendment to go in the wrong direction in this respect.
Liberty and autonomy - the FMA improperly intrudes on individual choices about intimate relationships.
Fairness - the FMA would permanently enshrine unfair treatment of people for arbitrary reasons.
Separation of church and state - the primary impetus for the FMA comes from people who openly avow their desire to put "the sanctity of marriage" into the Constitution. The Constitution does not sanctify anything, and it should not. Sanctity is a church function. Similarly, most opposition to same-sex marriage comes from a belief that churches should not recognize such marriages. That is not an issue that the Constitution can or should resolve. The Constitution as it now stands leaves it to each religion to decide what marriages to recognize.
Constitutional restraint - the Federal constitution has been amended 17 times since the Bill of Rights. It should not be changed when it is not necessary. There is no compelling reason to change the constitution over this issue, which is better left to state and federal legislation and the courts.
Civil unions - the Musgrove version of the amendment is being promoted as allowing civil unions, but the language of the amendment ("marriage or its incidents") could easily be read by the courts to prohibit civil unions as well. No court is likely to interpret a constitutional amendment as merely regulating the use of a single word; "marriage-like" relationships may well be swept within its prohibitions as well. Thus the FMA as currently proposed is a much more extreme measure than its proponents admit or most people would accept. Assurances from the proponents of the measure are worth little, since ---if it passes--- it is the courts that would open this Pandora's box and tell us what is inside.
Marriage - the FMA is anti-marriage. Marriage benefits society by providing a stable and predictable environment for children and for adults who form permanent commitments to each other. The FMA will not reduce the number of same-sex couples or the number of children being raised in families headed by same-sex couples, but it will force these families to exist outside the institution of marriage. It would make it impossible for states to take measures they see fit to protect the stability of these families.