What is the theme in God Emperor of Dune by Frank Herbert?

God Emperor of Dune is infused with sexuality, first because Leto II is preoccupied by perfecting the genetic program he has taken over from the Bene Gesserit, to perpetuate the one gene needed to prevent the extinction of humankind. He intends the latest Duncan Idaho to mate with Siona Atreides, but the ghola adamantly insists he is no one's stud. He resents having (perhaps) been mated on the night of his arrival on Dune with the houris assigned to tame him.