A Federal District Court judge has ruled Section 3 of the Defense of Marriage Act, which defines marriage as only between a man and a woman, is unconstitutional.

The relevant part of the ruling U.S. District Judge Joseph Tauro states:

This court has determined that it is clearly within the authority of the Commonwealth to recognize same-sex marriages among its residents, and to afford those individuals in same-sex marriages any benefits, rights, and privileges to which they are entitled by virtue of their marital status. The federal government, by enacting and enforcing DOMA, plainly encroaches upon the firmly entrenched province of the state, and, in doing so, offends the Tenth Amendment. For that reason, the statute is invalid.

The ruling seems to be based on the Tenth Amendment and the Due Process Clause of the Fifth Amendment. The basic idea being that the states have the power to regulate marriage, not the federal government.

Of course this is only the district court level so I expect there to be appeals to the Court of Appeals and eventually the US Supreme Court.

The ruling came from two cases: Gill v. OPM, brought by GLAD, and the Commonwealth of Massachusetts v. HHS, brought by Mass. Attorney General Martha Coakley.