A common misunderstanding is that a moral relativist rejects morality. However, just because someone is a moral relativist does not mean that he or she cannot have strongly held moral values. A moral relativist believes that any society can create and enforce whatever moral laws it needs in order to function and survive. So if some people feel the need for a right to abortion, pornography, divorce, contraception, same sex marriage, etc. in order to pursue life and happiness as they see it, without the inconvenience and unpleasantness of having to deal with criticism and restrictions from those who disagree with them, then they also believe it is their right to silence their critics by any means necessary to achieve their goals.
A nation that no longer respects Divine Law nor Natural Law is a nation in which there is no basis for any consensus as to what constitutes a good life or a good person. So it is that in a relativistic society there is no basis for any agreement between left and right, progressive and conservative, or secular and religious. There is only power politics and the principle that in a relativistic society might makes right.
This is what Pope Benedict meant by the “dictatorship of relativism”.