The arguement, a valid one, is that Mass's supreme court, only 4 of the 7 judges, decided the whole issue, because of the free faith and credit clause of the Constitution. In effect, the judges have removed this from the state's right to decide.
And I'll point out that they did the same with abortion.
It's best to leave everything as the status quo. There is likely going to be a backlash against the gay community because of this bloody marriage issue. Even Bush supports (or at least says so) Civil Unions.