Massachusetts is suing the United States government over the Defense of Marriage Act, the Federal law that defines marriage as a union between a man and a woman.
Massachusetts is claiming that the law interferes with the right of Massachusetts to define and regulate marriage as it sees fit. The law not only denies federal recognition of same-sex marriage, it gives states the right to refuse to recognize same-sex marriages performed in other states.
According to reports, the suit claims that the act "constitutes an overreaching and discriminatory federal law." The argument strongly comes down to States rights, with Massachusetts noting that before the act, the federal government recognized that marital status was the exclusive domain of the states.
Here's a local news report on the move: