The Bible has much to say about the sin of fornication. The word is used in several different ways, but the one I wish to focus on here is sexual sin. Fornication, in the sexual sense, is mainly sex between unmarried people, or between a married and unmarried person (also called adultery, see Mt 5:32, 19:9). It is interesting to note that in the lists of sins Paul mentions, fornication is listed first in most. Jesus also calls it an evil sin (Mt 15:19)(Mk 7:21). We are told to flee from fornication. Joseph is a wonderful example of this, literally running away when Potiphar’s wife tried to cause him to commit this sin (Gen 39:7-12). The Bible also warns that those who commit this sin shall not enter the kingdom of God (Eph 5:5)(Gal 5:21)(1 Cor 6:9).
(1 Cor 6:18-19) Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. (19) 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?
(1 Cor 6:13) Meats for the belly, and the belly for meats: but God shall destroy both it and them. Now the body is not for fornication, but for the Lord; and the Lord for the body.
(1 Cor 7:1-2) Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman. (2) Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
(Eph 5:3,5-6) But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints; (5) For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the Kingdom of Christ and of God. (6) Let no man deceive you with vain words: for because of these things cometh the wrath of God upon the children of disobedience.
(1 Th 4:3-5) For this is the will of God, even your sanctification, that ye should abstain from fornication: (4) That every one of you should know how to possess his vessel in sanctification and honour; (5) Not in the lust of concupiscence (passion), even as the Gentiles which know not God:
Here is how seriously God takes premarital sex:
(Deut 22:20-21) But if this thing be true, and the tokens of virginity be not found for the damsel: (21) Then they shall bring out the damsel to the door of her fathers house, and the men of the city shall stone her with stones that she should die: because she hath wrought folly in Israel, to play the whore in her father’s house: so shalt thou put evil away from among you.
We are also not to keep company with a fornicator:
(1 Cor 5:11) But now I have written you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer (abusive), or a drunkard, or an extortioner, with such an one, no not to eat. (Paul also speaks of this earlier in the chapter: 1 Cor 5:1-2)