Isn't marriage a state responsibility under your constitution? How would a federal law change anything?
Due to the Supremacy Clause of the Constitution, federal law trumps state law. If word comes from any of the 3 branches of government, in the form of a federal law, a supreme court decision, or an executive order, the state law loses. This, incidentally, is why gay marriage has never been brought to the Supreme Court. If someone tries to dispute the constitutionality of gay marriage in the Supreme Court and loses, then the Supreme Court ruling will override everyone else. Either all states would be forced to recognize gay marriage, or no states would. It's too big a risk for either side.