A reading of all Federal Law, such as income tax filing laws, inheritance laws, bankruptcy laws and Social Security laws all use the term "married" and that term is defined by the States and not by the Federal government. The Federal government cannot discriminate related to every single law that uses the term "marriage" by establishing a federal definition of what the federal government considers to be "legally married under State law" as that violates the State's Rights to define what "legally married" is.
If the benefit is based upon "Legally Married" under the statute then the federal government cannot come back and say, "Legally married EXCEPT FOR YOU" as that is discriminatory and a denial of equal protection under the law.

