The great marriage is a druid ceremony that takes place as part of the Beltane Festival (our May Day).  It was used by Viviane to ensure the loyalty of the high king to the druid religion.  In Arthur’s Kingmaking ceremony, he has to kill the king stag, armed with nothing but his wits and a dagger.  He succeeds, and he is brought to the cave were the great marriage is to take place.
  The Celtic or Druid religion is similar to the modern day Wiccan religion, in that
the worship a female Goddess, or the great mother.  Similar to Christianity, the Goddess figure is oter represented as a trinity: Mother, Maiden, and Crone.  In the great marriage ceremony, Druid Priestesses become one with the great mother, and represent her metaphorical and physically.  The sexual act that takes place in the ceremony is representative of the greater and no less sexual act which created the world, and all of humanity.
  Arthur does not recognize his sister, and she does not recognize him, until after
the ceremony is complete, and the child conceived.  The guilt instilled in him becuase of this incest is used to control him later on in the story.