The federal law banning gay marriage is unconstitutional because it interferes with the right of a state to define the institution and therefore denies married gay couples some federal benefits, a federal judge ruled Thursday in Boston

via Federal gay marriage ban is ruled unconstitutional  | ajc.com.

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. – 10th Amendment to the US Constitution