Illicit sex relations outside of Scriptural marriage. The Hebrew verb za·nahʹ and its related forms convey the idea of harlotry, immoral intercourse, fornication, or prostitution. (Ge 38:24; Ex 34:16; Ho 1:2; Le 19:29) The Greek word translated “fornication” is por·neiʹa. Regarding the meanings of por·neiʹa, B. F. Westcott in his book Saint Paul’s Epistle to the Ephesians (1906, p. 76) says: “This is a general term for all unlawful intercourse, (I) adultery: Hos. ii. 2, 4 (LXX.); Matt. v. 32; xix. 9; (2) unlawful marriage, I Cor. v. I; (3) fornication, the common sense as here [Eph 5:3].” Bauer’s Greek-English Lexicon of the New Testament (revised by F. W. Gingrich and F. Danker, 1979, p. 693) defines por·neiʹa as “prostitution, unchastity, fornication, of every kind of unlawful sexual intercourse.” Porneia is understood to involve the grossly immoral use of the genital organ(s) of at least one human; also there must have been two or more parties (including another consenting human or a beast), whether of the same sex or the opposite sex. (Jude 7) The  unlawful act of a rapist is fornication, but, of course, that does not make the person who is forcibly raped also a fornicator.

When God performed the first human marriage he said: “That is why a man will leave his father and his mother and he must stick to his wife and they must become one flesh.” (Ge 2:24) Here the standard set for man and woman was monogamy, and promiscuous sex relationship was ruled out. Also, no divorce and remarriage to another was anticipated.​—See DIVORCE.

In patriarchal society God’s faithful servants hated fornication, whether between single, engaged, or married persons, and it was considered a sin against God.​—Ge 34:1, 2, 6, 7, 31; 38:24-26; 39:7-9.

Under the Law. Under the Mosaic Law, a man committing fornication with an unengaged girl was required to marry the girl and to pay her father the purchase price for brides (50 silver shekels; $110), and he could not divorce her all his days. Even if her father refused to give him the girl in marriage, the man had to pay the purchase price to the father. (Ex 22:16, 17; De 22:28, 29) However, if the girl was engaged, the man was to be stoned to death. If the girl screamed when she was attacked, she was not to be punished, but if the engaged girl failed to scream (thereby indicating consent), she was also put to death.​—De 22:23-27.

The sanctity of marriage was emphasized by the law that punished with death a girl who married under the false pretense of being a virgin, having committed fornication secretly. If her husband falsely charged her with such a crime, it was regarded as bringing great reproach on her father’s house. For his slanderous action, the man was to be “disciplined” by the judges, perhaps by beating, and fined 100 silver shekels ($220), the money then being given to the father. (De 22:13-21) Prostitution of a priest’s daughter brought disgrace on his sacred office. She was to be killed, then burned as something detestable. (Le 21:9; see also Le 19:29.) Fornication between married persons (adultery) was a violation of the seventh commandment and merited the death penalty for both parties.​—Ex 20:14; De 5:18; 22:22.

If a man committed fornication with a servant girl who had been designated for another man, but who had not been redeemed or freed, punishment was to take place, but they were not to be put to death. (Le 19:20-22) Evidently this was because the woman was not yet free and in full control of her actions, as a free engaged girl would be. The redemption price had not yet been paid, or at least not fully paid, and she was still a bondservant to her master.

When the mercenary prophet Balaam could not bring a curse upon Israel by divination, he found a way to bring them under God’s displeasure by appealing to wrong desire for sexual relations. By means of the women of Moab he seduced the Israelites into practicing the filthy phallic worship of the Baal of Peor, for which 24,000 of the sons of Israel died.​—Nu 25:1-9; 1Co 10:8 (likely 1,000 heads of the people were killed and hung on stakes [Nu 25:4] and the rest were destroyed by the sword or the plague).

Forbidden to Christians. Jesus Christ restored God’s original standard of monogamy (Mt 5:32; 19:9) and showed the wickedness of fornication by classing it with murder, thievery, wicked reasoning, false testimony, and blasphemy. He pointed out that these come from within a man, from his heart, and defile him. (Mt 15:19, 20; Mr 7:21-23) Later, the governing body of the Christian congregation, comprised of the apostles and older men in Jerusalem, wrote to Christians in about 49 C.E., warning them against fornication, and placing it alongside idolatry and the eating of blood.​—Ac 15:20, 29; 21:25.

The apostle Paul points out that fornication is one of the works of the flesh, the opposite of the fruitage of the spirit of God, and warns that the practice of fleshly works will prevent an individual from inheriting the Kingdom. (Ga 5:19-21) He counsels that the Christian should deaden his body “as respects fornication.” (Col 3:5) In fact, he warns that it should not even be a topic of conversation among Christians, who should be holy. Similarly, the Israelites were not to mention the names of the pagan gods​—not that they would fail to warn their children about these gods, but they would not mention them with any esteem.​—Eph 5:3; Ex 23:13.

Fornication is an offense for which an individual may be expelled (disfellowshipped) from the Christian congregation. (1Co 5:9-13; Heb 12:15, 16) The apostle explains that a Christian committing fornication sins against his own body, using reproductive members for a perverted purpose. He is greatly affected spiritually in an adverse way, brings defilement into God’s congregation, and lays himself open to the danger of deadly sexually transmitted diseases. (1Co 6:18, 19) He encroaches on the rights of his Christian brothers (1Th 4:3-7) by (1) bringing uncleanness and  disgraceful folly, with reproach, into the congregation (Heb 12:15, 16), (2) depriving the one with whom he commits fornication of a clean moral standing and, if that one is single, of being clean when entering into marriage, (3) depriving his own family of a clean moral record, as well as (4) wronging the parents, husband, or fiancé of the one with whom he commits fornication. He disregards, not man, whose laws may or may not condone fornication, but God, who will exact punishment for his sin.​—1Th 4:8.

Symbolic Use. Jehovah God spoke of the nation of Israel in covenant relationship to him as “a wife.” (Isa 54:5, 6) When the nation became unfaithful to him, ignoring him and turning to other nations such as Egypt and Assyria for help and entering into alliances with them, Israel was like an unfaithful wife, an adulteress, a prostitute, one carrying on fornication promiscuously. (Eze 16:15, 25-29) Likewise, if Christians in a dedicated relationship to God, or professing such a relationship, are unfaithful by engaging in false worship or by being friends of the world, they are called adulteresses.​—Jas 4:4.

Concerning the symbolic meaning of por·neiʹa in certain texts, F. Zorell (Lexicon Graecum Novi Testamenti, Paris, 1961, col. 1106) says: “Apostasy from the true faith, committed either entirely or partially, defection from the one true God Jahve to foreign gods [4Ki 9:22; Jer 3:2, 9; Ho 6:10 etc.; for God’s union with his people was considered like a kind of spiritual matrimony]: Re 14:8; 17:2, 4; 18:3; 19:2.”​—Brackets his; 4Ki in the Greek Septuagint corresponds to 2Ki in the Masoretic text.

Babylon the Great, described in the Bible book of Revelation as a harlot, is a symbol of something religious. Her various sects, “Christian” and pagan, have claimed to be organizations of true worship. But she has consorted with the rulers of this world for power and material gain, and with her “the kings of the earth committed fornication.” Her unclean, filthy course of fornication has been detestable in God’s sight and has caused great bloodshed and distress in the earth. (Re 17:1-6; 18:3) For her course she will suffer the judgment of God on those practicing fornication, namely, destruction.​—Re 17:16; 18:8, 9.