Notice that the case is being brought before a state supreme court -- this is part of the strategy.
States define marriage, and they have a certain amount of cover from the Tenth Amendment and state sovereignty in these matters -- cover that former Supreme Court Justice William Brennan, in retirement, has been urging liberal "cause people" to make use of, to shelter from the more-conservative appointees to SCOTUS over the last 15 years or so.
This Massachusetts case follows cases in Vermont and Hawaii in which Evan Wolfson and Lambda tried to get the state supreme court to rule that homosexuals had the right to marry. That was Step One, getting marriage redefined in one state.
Step Two is, after Step One was accomplished, to sue in every other state in the Union to force them all to accept gay marriage (of course against the People's will -- that was the idea of going to court in the first place) under the Full Faith and Credit Clause of the U.S. Constitution.
In other words, the first state to accept marrying a man to his brother, or his dog, or a car, becomes the lowest common denominator in the definition of marriage, and the trendsetter for the nation.
Evil, huh? The Hawaiian case was mooted when the people woke up and circulated and passed a statewide referendum stomping gay marriage flatter than an empty beer can. The political ferment is still working in Vermont, and the outcome is cloudy -- but the legislature there decided on "civil unions" instead, which doesn't satisfy Wolfson (according to an interview he gave), because he and his playmates want the whole nine yards. They want gay marriages to be accepted as being every bit as solemnly moral as getting hitched by the Pope in St. Peter's Basilica. That is what they are after.