“And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity.” (Leviticus 20:17)
Nearly every society in history has had prohibitions against incestuous relations. Besides potential genetic problems, they almost always result in conflicts between members of the family, and emotional problems. Whether the sex act was completed or only looking or touching were involved, sexual activity between siblings is wrong, and has serious effects, whether full brother and sister or not. They were to be isolated from the people to prevent further problems. The responsibility rested solely on them.
“And if a man shall lie with a woman having her sickness, and shall uncover her nakedness; he hath discovered her fountain, and she hath uncovered the fountain of her blood: and both of them shall be cut off from among their people.” (Leviticus 20:18)
Knowing the increased risk of infections to both parties or of spreading it to others from sexual contact during the menstrual period, those who deliberately ignored the risks were to be isolated from other people to protect them.
"And thou shalt not uncover the nakedness of thy mother's sister, nor of thy father's sister: for he uncovereth his near kin: they shall bear their iniquity.
And if a man shall lie with his uncle's wife, he hath uncovered his uncle's nakedness: they shall bear their sin; they shall die childless.
And if a man shall take his brother's wife, it is an unclean thing: he hath uncovered his brother's nakedness; they shall be childless.” (Leviticus 20:19-21)
Nudity or sexual relations between other relatives has the same potential for causing conflict and emotional problems within the family. They were responsible for their part in the activity, and God would not bless such involvement with children, which would only increase the conflict.
“Ye shall therefore keep all my statutes, and all my judgments, and do them: that the land, whither I bring you to dwell therein, spue you not out. And ye shall not walk in the manners of the nation, which I cast out before you: for they committed all these things, and therefore I abhorred them.” (Leviticus 20:22-23)
As a result of Adam’s sin in the Garden of Eden, man had a sense of right or wrong, a conscience. All these behaviors stimulated that conscience, but people often find pleasure in the sense of guilt and getting by with something, and eventually it no longer bothers them. After a while they have to do something else to get the same sense of guilty pleasure. The original inhabitants of the land had practiced all these sins, accepting them as an alternative lifestyle, and causing God to abhor them. It finally reached a point where God evicted them from the land, much a landlord might evict tenants who used a rental property for selling drugs or prostitution. In the next lease agreement, the landlord might well specify that using the home for illegal activities was grounds for eviction.
In much the same fashion, God warned Israel that indulgence in such activities would result in their eviction. Few question a landlord’s right to set such a standard, yet many people challenge God’s right to do so.
“But I have said unto you, Ye shall inherit their land, and I will give it unto you to possess it, a land that floweth with milk and honey: I am the LORD your God, which have separated you from other people.” (Leviticus 20:24)
God had promised to allow Israel to occupy the land he had taken from the previous occupants. It was a good land, and God had chosen them to occupy it rather than letting whoever wanted to.
“Ye shall therefore put difference between clean beasts and unclean, and between unclean fowls and clean: and ye shall not make your souls abominable by beast, or by fowl, or by any manner of living thing that creepeth on the ground, which I have separated from you as unclean. And ye shall be holy unto me: for I the LORD am holy, and have severed you from other people, that ye should be mine.” (Leviticus 20:25-26)
Just as a landlord would expect some form of remuneration for the use of his property, God made some requirements for Israel. They were to distinguish between animals which were potential disease threats and those which were not and avoid contact with those which were. They were to live a life that was acceptable to God, keeping themselves in a holy state because of what God had done for them.
Considering that most of the demands would benefit them even more than it would God, it seems a no brainer to practice them, even if he didn’t require it, kind of like a landlord asking that the house be kept clean. Unfortunately, many people don’t consider how much they benefit from such demands.
“A man also or woman that hath a familiar spirit, or that is a wizard, shall surely be put to death: they shall stone them with stones: their blood shall be upon them.” (Leviticus 20:27)
Anyone who practiced occultic arts such as séances or palm reading, or who used secret rituals to try to invoke the power of other gods to control the world around them was to be put to death. They were essentially trying to replace God.