Genderless Marriage: Just or Unjust law?

In the debate over whether to redefine marriage I haven't heard much discussion over the justness of redefining marriage. An interesting place to look for the answer is the words of the leader of the civil rights movement Martin Luther King, Jr. 

I came across his famous "Letter from a Birmingham Jail."  While in jail, he penned his famous letter discussing segregation laws and the nature of laws, their justness and unjustness.

He wrote:

One may well ask: "How can you advocate breaking some laws and obeying others?" The answer lies in the fact that there are two types of laws: just and unjust. I would be the first to advocate obeying just laws. One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws. I would agree with St. Augustine that "an unjust law is no law at all." 

Now, what is the difference between the two? How does one determine whether a law is just or unjust? A just law is a man-made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law.

How does this apply to marriage?  First, it's clear that marriage according to God's law, is one man and one woman.  Therefore, to redefine marriage and force people to recognize, endorse, and support it would be unjust.

So Minnesota legislators need to realize what they are being asked to do.  They aren't "eliminating discrimination"  and promoting "marriage equality", instead they are instituting an unjust law.