It's about whether people should be able to use the Constitution to impose their moral foundation on another group of people. Someone else in this thread made the argument that they are free to vote their conscience. But THAT is precisely why the rights of a group of people should NOT be put up for a popular vote! If people had just been allowed to vote their conscience in the 1960s, we'd still have Jim Crow laws. Hell, in South Carolina (1998) and Alabama (2000) roughly 40% of the people voted that they wanted the interracial marriage bans left in their Constitutions, even though they had been declared invalid by the Supreme Court in 1967.
The U.S. Constitution was set up so that the majority rules, but the rights of the minority are protected. The 14th Amendment is there so that nobody can be legislated into "second class citizen" status, be it African-Americans, Hispanics, gays, lesbians, Jews, Hindus, Wiccans, or Muslims.
Eventually the amendments against gay marriage will not stand up to the 14th Amendment.
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