One thing that many of my high school students understand as common sense, but many adults do not, is that not everything that is legal is moral. In fact, pro abortionists consistently return to the fact that abortion is legal to justify the killing of an unborn child. The conversation inevitably looks something like this:
Pro lifer: "Abortion takes the life of a human person."
Pro abortionist: "Abortion is legal."
Pro lifer: "Slavery was legal once."
Pro abortionist: "That's different."
Why would someone uphold that slavery in itself is immoral and uphold that laws against it were a correct choice of action? Because they rightly see that it is in its very nature unjust, and thus a law that permits it is unjust. What makes a law for or against something unjust?
Dr. King in this same letter proposes correctly that a law is unjust if it does not square with the law of God. This law of God, the Natural Law, is so called because it governs the heart of every human person insofar as they allow it to. Thus any human law that contradicts this law of God is unjust.
In fact, this is why I can take the words of Dr. King and apply them to abortion: both segregation and abortion are unjust. They relegate human persons to something less, to the status of things, or use them for their own purpose. This is also why Dr. King can use the example of the holocaust to condemn segregation. Both relegate human persons to something less, to the status of things.
Abortion should be illegal because it is immoral; it has not been made moral because it is legal.