Jamieson, Fausset and Brown on Leviticus 18:30

30. Therefore shall ye keep mine ordinance, that ye commit not any one of these abominable customs--In giving the Israelites these particular institutions, God was only re-delivering the law imprinted on the natural heart of man; for there is every reason to believe that the incestuous alliances and unnatural crimes prohibited in this chapter were forbidden to all men by a law expressed or understood from the beginning of the world, or at least from the era of the flood, since God threatens to condemn and punish, in a manner so sternly severe, these atrocities in the practice of the Canaanites and their neighbors, who were not subject to the laws of the Hebrew nation.