Sexual intercourse between a married person and someone who is not his or her mate is adultery.
The Ten Commandments contain the prohibition against adultery: “You shall not commit adultery” (Exodus 20:14). The reason is simple: Marriage is the foundation of society, and with it comes the responsibility of child rearing. Casual sex outside marriage not only imperils marriage but also destroys the paternal or maternal feeling for the children of the marriage, and blurs the lines of inheritance and family, clan and tribal relationships.
Fornication is sex between two unmarried people. The apostle Paul said this is a sin against the body. He commands Christians to flee fornication as a sin against self and God, for the believer’s body is the temple of the Holy Spirit (see I Corinthians 6:18-19). Paul says that if a believer takes his body and joins it to a harlot (or someone who is immoral), he is joining Jesus Christ to that person (see I Corinthians 6:15-16).
It is very important to understand that neither fornicators nor adulterers will enter the kingdom of heaven (see I Corinthians 6:9-10). In today’s world, the term fornication is rarely used and immorality between unmarried people is commonly accepted as a lifestyle. But immorality, however commonplace, is a sin that will keep millions of people out of heaven, unless they repent.