1 Corinthians 6:9-10 

Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.

Webster defines fornication as voluntary sexual intercourse between two unmarried persons. How does the bible define the word? Fornication comes from the Greek word pornea. It means sexual immorality. You also see the word pornea in the word pornography. 

How does the bible define sexual immorality? 

Sex outside of marriage has become acceptable in the post-modern world.

Objections:

* Premarital sex wasn't forbidden in the bible until 30-100 AD

* Premarital sex isn't forbidden at all 

* Fornication is used in the context of being unfaithful to your husband or wife

* It is ok to have sex if you are in love as did Adam & Eve

Matthew 5:32  

But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery.

In the above context, it is not exclusively speaking of premarital sex, but rather sex with another partner other than your husband or wife.  

Romans 1:29-31

“Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, without understanding, covenant breakers, without natural affection, implacable, unmerciful:

1 Corinthians 7:2 

“Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.“

In the above context sex outside of marriage is sin. 

Galatians 5:19-20

“Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies…“

The above context Here we see a distinction between adultery and fornication. 

Below, God forbids all kinds of sexual activity: incest, menstrual sex, adultery, homosexuality, and bestiality. 

Levititcus 18:6-23 - Incest 

“None of you shall approach anyone who is near of kin to him, to uncover his nakedness: I am the LORD. The nakedness of your father or the nakedness of your mother you shall not uncover. She is your mother; you shall not uncover her nakedness. The nakedness of your father’s wife you shall not uncover; it is your father’s nakedness. The nakedness of your sister, the daughter of your father, or the daughter of your mother, whether born at home or elsewhere, their nakedness you shall not uncover. The nakedness of your son’s daughter or your daughter’s daughter, their nakedness you shall not uncover; for theirs is your own nakedness. The nakedness of your father’s wife’s daughter, begotten by your fathers he is your sister you shall not uncover her nakedness. You shall not uncover the nakedness of your father’s sister; she is near of kin to your father. You shall not uncover the nakedness of your mother’s sister, for she is near of kin to your mother. You shall not uncover the nakedness of your father’s brother. You shall not approach his wife; she is your aunt. You shall not uncover the nakedness of your daughter-in-law she is your son’s wife you shall not uncover her nakedness. You shall not uncover the nakedness of your brother’s wife; it is your brother’s nakedness. You shall not uncover the nakedness of a woman and her daughter, nor shall you take her son’s daughter or her daughter’s daughter, to uncover her nakedness. They are near of kin to her. It is wickedness. Nor shall you take a woman as a rival to her sister, to uncover her nakedness while the other is alive.

Menstral Sex 

“Also you shall not approach a woman to uncover her nakedness as long as she is in her customary impurity.

Adultery

Moreover you shall not lie carnally with your neighbor’s wife, to defile yourself with her.

Homosexuality

You shall not lie with a male as with a woman. It is an abomination.

Bestiality 

Nor shall you mate with any animal, to defile yourself with it.

 Matthew 5:27-30

“You have heard that it was said to those of old, ‘You shall not commit adultery.’ But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.

Masterbation

For children this is a natural inclination to what feels good. As children get older and grown in knowledge this involves sexual fantasy and/or pornography, and so it is sin. It is the current spiritual epidemic we face today with in out technological society. Notice how Jesus talks about the eye (fantasy) to the physical action (right hand). The progression of lust can get progressively worse and more addictive. He is using hyperboly to show how severe sin is to the Holy God and how all of us fall short of the glory of God. 

God set boundries so that our joy may be complete.