Without government, marriage is a religious ceremony only and, while quite important in that respect, would confer no special privileges. Since government defines what privileges are granted to married couples, it makes sense that government gets to define what a marriage is. And in the U.S., the government is supposed to do the will of the people.
Now, in this case the Justices of the Massachusetts Supreme Court have decided that the law defining marriage as between men and women only in Massachusetts conflicted with the Commonwealth's equal protection laws, and then further decided the conflict in favor of requiring same-sex marriages. I believe they were wrong on both counts. My guess is, so will the Commonwealth's citizens.