Jesus said that the only valid cause for divorce is marital unfaithfulness (Matthew 19:8-9; Mark 10:2-12). Biblical scholars disagree as to whether marital unfaithfulness refers only to physical adultery, or whether it includes other types of unfaithfulness in marriage. This has been the subject of much debate within the church.

Porneia, the word used for marital unfaithfulness in Matthew 19:9, can be used not only of sexual sin in the Bible, but is also used metaphorically for spiritual unfaithfulness (Strong’s Concordance). In the same way, the spiritual unfaithfulness of Israel is described by the Hebrew word, zanah, meaning adultery, or figuratively, idolatry, in numerous instances in the Old Testament (Exodus 34:16; 2 Chronicles 21:11,13; Isaiah 1:21; Jeremiah. 3:8; and Ezekiel 16 and 23).

The following verses also argue against a strictly physical interpretation of marital unfaithfulness:

1. In Matthew 5:27-28, Christ taught that adultery can occur in the heart and mind, not just in the physical.
   
2. Christ taught that divorce was permitted for hardness of heart (Matthew 19:8).
   
3. In 1 Corinthians 7:15, Paul allows divorce if an unbelieving spouse wants to abandon the relationship. This scripture does not mention physical adultery.

In Jeremiah 3:8 we see that when the northern tribes of Israel refused to turn from their sin, God divorced them and proclaimed them to be adulterous (using the same word as for "marital unfaithfulness"). Domestic violence is sin; could it also be a type of marital unfaithfulness? Certainly it indicates an unfaithfulness to the basic marital vows to love, honor, and protect.

Why doesn’t the exception for marital unfaithfulness appear in Jesus’ teachings in Mark 10:11-12 or Luke16:18? Here are explanations offered by Guy Duty, a respected biblical scholar, and Herb Vander Lugt, researcher with Radio Bible Class:

• Guy Duty: "An exception in a fuller account must explain a briefer one."
  

• Herb Vander Lugt: "This exceptive clause was omitted in the other Gospels simply because it wasn’t needed. All first-century readers—Jewish, Greek, and Roman—agreed that sexual infidelity was legitimate grounds for divorce. Commands that have well-known or obvious exceptions are often stated without a repetition of the exceptions. For example, Paul in Romans 13:1-7 told us to obey governing authorities and states no exceptions. Peter in his first epistle (2:13-16) told us to submit to "every ordinance of man for the Lord’s sake" and gave us no exceptions. But both know full well the principle expressed by Peter and recorded in Acts 5:29, We ought to obey God rather than men.’ They assumed their readers did too."