I was getting ready to make the statement that, based on the Tenth Amendment, DOMA was unconstitutional and that nothing that the federal government can constitutionally do is at all related to marital status. But then I started thinking, what about income tax and the different filing statuses? Does the federal government have any constitutional powers that would take the definition of marriage into account, even if that definition would not be binding on the states?
DOMA had two parts. Only the first was declared unconstitutional (definition of marriage); the second was upheld (States behind allowed to reject marriages from other States). The second provision of the law was simply a restatement of federalism and important. I would assume that a person could claim married on a federal tax return but if their State does not recognize gay marriage then they could not claim that on a State return.