I have heard the question several times but just decided to answer it since the answer is very clear in the Bible. Many people are not aware that Adam and Eve had daughters (Gen 5:4). When Adam was 130 years old he had a third son called Seth (Gen 5:3). After Cain killed Abel, he was now concerned about his own safety since he only knew about the world around him. He feared for what was out there that he did not know. Later, Cain found a woman and married her (Gen 4:14-16). A question quickly arises: where did Cain find the woman he married. It is probable that he got one of his sisters who had wondered into his territory unknowingly. Since there is no time chart as to when he got married and by then Adam and Eve had had sons and daughters.
At this time there was no ordinance against siblings having sex together. As such there was nothing like incest. So it is tacitly probable that the daughters and sons had sex together to procreate. Through them there were now many people until the flood (Gen 7:1). The flood had destroyed the world that was procreated by the sons and daughters of Noah.
Noah’s household which consisted of only his three sons and their wives and him and his wife was in the Ark. After the flood, the normal procreation continued and again there is nothing established as incest.
This practice will continue as we realized that Abram (Abraham) married his own sister Sarah (Gen 20:12). The ordinance of God and the moral law are in the same strength in that they transcend cultures and people. However the ceremonial and religious laws do not. Yet there is one set of law that is left: civil law. The civil laws are authenticated with the society and culture at a time. That is; though polygamy is not condemned in the Bible in countries where polygamy is prohibited the citizens living in that country must obey that law. As such in the days of Israel now they enacted a law in Israel against incest: “None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD” (Lev 18:6). In Leviticus 20: 17 the law is amended with consequences established. Then later the law is amended and a spiritual dimension attached to it: and a curse against incest: Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother. And all the people shall say, Amen (Deut 27:22).
The amendments to those civil laws neither equate to abrogation nor the unauthenticity of the Bible. What is does is confirm the sociologists assertion that no society is static. Take for instance in most states there was no major law guiding internet usage. But with time and the types of crimes committed new laws are set and a new approach taken to the situation. But then why were neither the children of Adam and Eve nor Lot (Gen 19:36). The Apostle Paul explained the dynamics of the Law to the Romans. People who lived before the law (moral law) was enacted were not judged by Law (Rom 2:12). Those who did not have the law were judged by their consciences (Rom 2:15). Only those under the law bear the consequences in the Law (Rom 3:19). For until the law was given there was sin but that sin was not judged by the Law. Sin began being judged by the Law only when the Law was given (Rom 5:13).
Therefore since the children of Adam and Eve and those of Lot did not yet have the La they could not be judged by the law. It is simple with us nowadays. When abortion was not illegal in some states no one was being judged by that. Does that mean it was not evil? It was but there was just no law judging it.
Now after the law has been established, defaulters are lawfully punished. That is why when Amnon (One of David’s son) raped his step sister Tamar (2 Sam 13:32) he paid with his life as a fulfillment of the curse that was earlier enacted in Deuteronomy 27:22.
However, if you have committed incest, know that where sin abounds grace does abound (Rom 5: 20). If you will confess your sins, God is faithful and just to forgive your sins, and to cleanse you from all unrighteousness. However, if you do not there is a curse associated with it in addition to the sentence of the local law in which you live. After all most countries and places have incest as a crime.
Until then, the Law only works where it exists.
Prince & PA Hamilton Ayuk
If relationship was only sex then Hollywood would be in a better predicament but unfortunately, if love was shoes, then Hollywood would charter all sizes.