You and the supporters have it backwards.
Congress shall make no law respecting and establishment of religion. . .
Marriage is an establishment of religion.
Government cannot protect or attack marriage. It is an institution of the church. Judges affirming a new definition of marriage or legislative votes all violate the civil rights found in the first amendment.
This is the intrusion. Will is probably right about DOMA but that is also why all of the government changes to marriage definitions are unconstitutional as well.
Separation of church and state NEVER protects the church. It apparently always protects the State and its ideological preferences.
Congress shall make no law respecting and establishment of religion. . .
Marriage is an establishment of religion.
But I agree with you. Government should get out of the marriage business. Probably my crappy writing that led you astray.