The most important relationship on this earth (aside from mankind's relationship with God) is the relationship between a husband and his wife. From the beginning God has set forth this principle (Gen. 2:18-24). The husband and wife relationship is a relationship which is to not only take priority over the relationship between father and mother (Gen. 2:24; Mt. 19:5), but also over the relationship between parents and children (it is the parents who are joined together as one flesh, not the children and parents). It would then be logical to conclude that since marriage is so important, the unlawful breaking of that relationship is not lightly taken by God.

What Is "Fornication"?
"Fornication" (from the Greek, porneia) is defined by Thayer as "illicit sexual intercourse in general." This definition is placed within the broadest sense of the meaning. Too often, people limit "fornication" to "adultery," which, according to the Oxford Dictionary, means voluntary sexual intercourse between a married person and a person who is not his or her spouse. It is true that fornication includes adultery, but does not necessarily limit fornication to adultery. Every form of literal fornication involves the use of the body (1 Cor. 6:18-20). Therefore, fornication includes a variety sexual sins such as adultery, homosexuality, bestiality, pre-martial sex, etc.

Why Is It the Only Scriptural Grounds for Divorce and Remarriage?
Jesus said in Matthew 19:9, "Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery...." This statement was so profound that those who were listening to Jesus said in response, "If the case of the man be so with his wife, it is good not to marry" (Mt. 19:10). It should not surprise the reader that the response was startling, and seemingly negative. The setting in which Jesus answered the Pharisees' questions was a setting of fake sincerity. Verse 3 reveals to the reader that the Pharisees came to Jesus "tempting him."

The idea of the Pharisees was to trap Jesus in taking sides with one of two Jewish schools: the school of Shammai (which taught that the only cause for a man to put away his wife was for whoredom) or the school of Hillel (which taught that a man could put away his wife for practically any reason imaginable).1 However, Jesus does not "take sides" as the tempting Jews had assumed. Instead, He takes their question back to the beginning (Gen. 2:21-24) and continues to preach to them that breaking a husband and wife relationship violates God's original commandment. Although God allowed Moses ("because of the hardness of your hearts," Mt. 19:8) to record in Deuteronomy 24:1-4 conditions that included divorce, this was not a command given by Moses. The Pharisees misapplied a passage in order to trap Jesus in His own words.

"Except it be for fornication" means exactly what it says. There is no hidden meaning or metaphorical device used in Matthew 19:9. The only Scriptural reason for a man to divorce a woman or a woman to divorce a man is for fornication.

Conclusion
The Bible clearly teaches that divorce and remarriage for any other cause than fornication is wrong. Jesus, speaking to the Samaritan woman in John 4, told her that the man whom she was currently with was not her husband. John the Baptist rebuked Herod for the woman which he unlawfully had (Mt. 14:3-4). Even during the time of Ezra many Israelites had unlawful wives. However, these Israelites were taught their wrongs and put away their unlawful wives (and some cases even involved children, Ezra 10:18-44). God wants us to be as faithful to Him as a man is to his wife and a woman is to her husband. The church is the bride of Christ (Eph. 5:22-32), and Christians are in the church (Acts 2:47 cf. 1 Cor. 12:13). For a Christian to go against Christ is for the Christian to commit spiritual fornication with the world. If one spouse can divorce the other spouse for any cause and still have authority to remarry, then why should the Christian worry about spiritually fornicating against God?

End Note
1 Adam Clarke's Commentary. Ed. Gary Gallant. Online Bible CD. Version 2.00.02. Winterbourne, Ontario: Larry Pierce, 2005.