leviticus 18:25 And the land is defiled: therefore I do visit the iniquity thereof upon it, and the land itself vomiteth out her inhabitants. - Free Bible Online

      25. therefore I do visit the iniquity thereof upon it; and the land itself vomiteth out her inhabitants--The Canaanites, as enormous and incorrigible sinners, were to be exterminated; and this extermination was manifestly a judicial punishment inflicted by a ruler whose laws had been grossly and perseveringly outraged. But before a law can be disobeyed, it must have been previously in existence; and hence a law, prohibiting all the horrid crimes enumerated above--a law obligatory upon the Canaanites as well as other nations--was already known and in force before the Levitical law of incest was promulgated. Some general Iaw, then, prohibiting these crimes must have been published to mankind at a very early period of the world's history; and that law must either have been the moral law, originally written on the human heart, or a law on the institution of marriage revealed to Adam and known to the Canaanites and others by tradition or otherwise.

JFB.