Fornication is sexual intercourse by two people who are not married to each other. This sin includes adultery which is sexual intercourse by a married person with someone who is not their mate. Fornication also includes sexual intercourse between people who are not married. It includes sexual deviation such as homosexuality [with someone of the same sex] and incest.
Fornication can be a Biblically permitted reason for divorce:
But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced commiteth adultery. (Matthew 5:32)
Fornicators will not inherit the Kingdom of God:
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators..shall inherit the kingdom of God. (I Corinthians 6:9-10)
The Bible tells us to abstain [refrain] from fornication:
For this is the will of God, even your sanctification, that ye should abstain from fornication. (I Thessalonians 4:3)
Nevertheless to avoid fornication, let every man have his own wife, and let every woman have her own husband. (I Corinthians 7:2)
Neither let us commit fornication, as some of them committed... (I Corinthians 10:8)
The body is not intended for fornication because it belongs to the Lord. For this reason, you must flee fornication:
...Now the body is not for fornication, but for the Lord, and the Lord for the body...
Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
What? Know ye not that your body is the temple of the Holy Ghost which is in you which ye have of God, and ye are not your own?
For ye are bought with a price; therefore glorify God in your body, and in your spirit, which are God's. (I Corinthians 6:13,18-20)
It is your responsibility to mortify fornication:
Mortify therefore your members which are upon the earth...fornication... (Colossians 3:5)
Fornication is not to even be named among believers:
But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints. (Ephesians 5:3)
If a person continues in fornication, he finally will be given over to it totally. According to Romans l, it can even lead to homosexuality. Eventually, his conscience will no longer be bothered by it:
For this cause God gave them up...Being filled with all unrighteousness, fornication...(Romans 1:26,29)