I once came across an online science fiction story dealing with the relativity of morality.
In it, humans on an interstellar mission come across a kind of insectoid species with crystaline bodies (if my memory serves me well) that have a moral code in which everything has to be sacrificed for the collective. Each group find each others morals repulsive and the insectoid species wants to exterminate the humans to convert the entire universe to their vision of morality.
While being in a kind of mexican stand off, a third species appears that has a more liberal morality than the humans and communicates through means of sex. Everything revolves around pure pleasure and the exchange of bodily fluids in their vision. They also come into conflict with the humans and insectoids over their views on morality.
Does this ring a bell to anyone?