For I have chosen him, that he may command his children and his household after him to keep the way of the Lord by doing righteousness and justice, so that the Lord may bring to Abraham what he has promised him.—Genesis 18:19
We keep hearing from confessing Christians the appeal to biblical justice in the question of marriage equality and the like. I think beneath the fundamental redefinition of marriage to something both foreign to and against the Scriptures is the fundamental redefinition of justice, as well.
Justice in the Bible is not some nebulous fairness or sense of altruistic equality. It is the spreading dominion of the righteousness of God. (Note how often justice is paired with righteousness in the Bible.) When God calls us to "do justice," he is calling us to reflect the life-giving, culture-flourishing, gracious abundance that is in keeping with his glory. Justice includes care for the poor and hungry and sick, for instance, because it is a reflection of God's righteousness to address the effects of the fall on individuals and systems with gracious provision. In a sense, justice is taking the prelapsarian mandate (Gen. 1:28) into the postlapsarian world. Justice is grounded in the harmonic Eden and aims at the shalom of the new Jerusalem (2 Chron. 9:8, Isaiah 33:5). Justice is sourced in the righteousness of God (Job 37:23), the holiness of God (Deut. 32:4). Therefore, biblical justice is only superficially fairness but more deeply the express manifestation of the righteousness of God.
In that sense, it is not justice for the state to sanction same-sex marriage. It is in fact injustice, because it sanctions as "righteous" what God has called unrighteous. The Bible calls the failure to do justice a "perversion of justice." Even in seeking to do justice, then, when Christians disconnect biblical justice from the concept of the kingdom of God, they are perverting justice. We are told multiple times in the Scriptures who will not inherit the kingdom of God. It is a rejection of the righteousness of God, therefore, to say those very same excluded will be included.
Christian culture has adopted the imbecile habit of thinking conceptually as the world does. Thus love becomes primarily a romantic feeling disconnected from the holy God who is love and the "definition" we see in 1 Corinthians 13. Peace becomes merely the absence of conflict or judgment or disapproval, functionally disconnected from the holy Lord who is himself peace (Eph. 2:14). Joy is not found in the Lord but instead following your bliss, doing what fulfills you, finding what makes you happy, etc. And justice becomes giving people what they want because, after all, it's only fair.
But the Scriptures do not lend us these amorphous virtues to be shaped however the cultural winds are blowing. All of the biblical virtues are embodied in Christ. They have a shape. It is not "keeping the way of the Lord" to call evil good (Mal. 2:17). Or just.