i) The Constitution is silent on marriage. There's no enumerated Constitutional right to homosexual marriage. After all, the Constitutional doesn't even say heterosexual marriage is a Constitutional right.
According to the 9th and 10th amendments, that means any legal or civil right to marriage must be enacted at the state level:
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.
The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.
For the Supreme Court to decree a Constitutional right to homosexual marriage violates Federalism. That falls outside the jurisdiction of the Supreme Court. That's a state issue.
ii) At best, the Supreme Court might strike down anti-miscegenation laws based on the 14th amendment. At least in that instance there's a general Constitutional principle in play: the 14th amendment was designed to overrule state laws discriminating against blacks.
iii) Furthermore, even if a given state passed a statue legalizing homosexual marriage, it would have to include religious exemptions. For state and federal laws don't nullify the first amendment. State and federal laws cannot violate the civil liberties accorded citizens in the first amendment. In case of conflict, the Bill of Rights takes precedence.
iv) That's true for private citizens and public employees alike. Surrending your Constitutional rights is not a condition of gov't employment.