Doing a study of the Book of Leviticus without touching on sexual sin would be like ignoring an elephant in the room. It just can't be done.
So...without further ado...
Chapters 18 and 20 get down to the nitty-gritty of sexual sin. I mean, nothing is left out! All of man's sexual depravity is spelled out in black and white, folks. Why? Because God knows what people do in secret and He is not happy about it. He makes that abundantly clear in these chapters. Some sexual offenses were worthy of death in God's eye!! (See also Ex. 22:19)
Sexual purity was always God's plan. In Genesis 2 God introduces us to that plan. Verses 24-25 are key:
"For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh. The man and his wife were both naked, and they felt no shame."
One flesh. That means sex! Adam and Eve had sex in the garden, just as God planned, blessed and ordained. They felt no shame in their nakedness. They felt no shame in coming together as a man and his wife are made to do. However, Leviticus makes it abundantly clear that something went terribly wrong after the fall of mankind. People took the beautiful gift of sex and made it perverted and sinful.
In Matthew 15:19 Jesus spoke about sexual immorality, including adultery, when he was teaching the disciples about what it means to be unclean. The Pharisee's were in an uproar because Jesus' followers didn't wash their hands before they ate, but Jesus makes it clear that the things that make a man unclean are due to what is in his heart rather than what is on his hands.
In Romans 1:24-25 Paul writes, "Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. They exchanged the truth of God for a lie."
In Romans 6:13 he says, "Do not offer the parts of your body to sin, as instruments of wickedness."
In 1 Cor. 6 he makes it clear our bodies belong to God and are not meant for sexual immorality.
In Ephesians 5:3 he says there should not be even a hint of sexual immorality if we claim to belong to Jesus Christ.
Finally, Revelation 21:8 and 22:15 make it very clear what will happen to those who choose sexual sin over God:
"But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts--their place will be in the fiery lake of burning sulfur. This is the second death."
"Outside [of heaven] are the dogs, those who practice magic arts, the sexually immoral, the murderers, the idolaters and everyone who loves and practices falsehood."
Beginning in Genesis 1 and going all the way through Revelation 22, you will find only one category of people who God blesses sexually: A man and his wife. (Genesis 1:28) Any sexual activity outside of the marriage between a man and his wife is not blessed by God. God created us and He created us as sexual beings, but He did not give us open permission for any and all sexual relationships.
Remember...you can't hide anything from God!
Not even an elephant.