Many of those who support the FMA claim that they are doing so only to prevent unelected judges from foisting unpopular decisions on democratic majorities. "It's not that we oppose gay marriage," amendment proponents seem to be saying, "it's just that we don't like this particular method of achieving them." I've written elsewhere about the problems associated with judicial activism ("Dwarfing the Political Capacity of the People?: The Relationship Between Judicial Activism and Voter Turnout," 1840-1988. Polity 25 (Summer 1993): 633-646), so I would have much less of a complaint if they were offering an amendment to scale back the power that courts have to excercise judicial review.
But that's not what's going on here. The FMA seeks to limit the judicial activism in only one instance--when courts interpret constitutional equal protection provisions in favor of gay and lesbian couples. That's not principle, that's prejudice.