31 "It was also said, 'Whoever divorces his wife, let him give her a certificate of divorce.'
32 But I say to you that every one who divorces his wife, except on the ground of unchastity (porneia), makes her an adulteress; and whoever marries a divorced woman commits adultery."
For one thing, it is unclear what the word porneia means. Baker's Evangelical Dictionary of Biblical Theology says [emphasis mine]:
In later Judaism and New Testament times the word broadened to include adultery, incest, sodomy, unlawful marriage, sexual intercourse in general, and any sexual behavior that deviates from accepted social and religious norms. Usage in New Testament contexts does not change these options. The argument of Matthew 5 (and Matt. 19) does not provide sufficient data to limit the usage of porneia in this context to one specific meaning. Porneia is perhaps broad in its reference to illicit sexual intercourse in keeping with the breadth of the Hebrew phrase ervat dabar (cf. Deut 24:1). Some form of illegitimate extramarital sexual intercourse is conveyed by the term.There is a great variety in the ways in which porneia is translated in our many versions of the Bible (keep in mind "unlawful marriage" as a possibility as well):
- "unchastity": NAS, NRS, RSV
- "fornication": ASV, KJV, TMB, DRB, Complete Jewish Bible, Darby, Vulgate ("fornicationis")
- "sexual immorality": ESV, NKJV, HCSB, TNIV, Hebrew Names Version, WEB
- "marital unfaithfulness": NIV, NLT, GNT, NIRV, GOD'S WORD Translation
- "sexual promiscuity": The Message
- "adultery": NCV
- "the loss of her virtue": Bible in Basic English
- "whoredom": YLT
- "lewdness": Webster's Bible
Mt 15:19 For out of the heart proceed evil thoughts, murders, adulteries (moicheia), fornications (porneia), thefts, false witness, blasphemies
Paul also makes this distinction in Gal 5:19. This could mean that porneia and moicheia are not the same thing.
What does porneia mean then? Baker's Dictionary, which I cited earlier, lists "unlawful union" as a possible meaning. The Navarre Commentary elaborates upon this possible definition:
31-32. The Law of Moses (Deuteronomy 24:1), which was laid down in ancient times, had tolerated divorce due to the hardness of heart of the early Hebrews. But it had not specified clearly the grounds on which divorce might be obtained. The rabbis worked out different sorts of interpretations, depending on which school they belonged to--solutions ranging from very lax to quite rigid. In all cases, only husband could repudiate wife, not vice-versa. A woman's inferior position was eased somewhat by the device of a written document whereby the husband freed the repudiated woman to marry again if she wished. Against these rabbinical interpretations, Jesus re-establishes the original indissolubility of marriage as God instituted it (Genesis 1:27; 2:24; cf. Matthew 19:4-6; Ephesians 5:31; 1 Corinthians 7:10).The RSVCE has this note on porneia [emphasis mine]:
The phrase, "except on the ground of unchastity", should not be taken as indicating an exception to the principle of absolute indissolubility of marriage which Jesus has just re-established. It is almost certain that the phrase refers to unions accepted as marriage among some pagan people, but prohibited as incestuous in the Mosaic Law (cf. Leviticus 18) and in rabbinical tradition. The reference, then, is to unions radically invalid because of some impediment. When persons in this position were converted to the True Faith, it was not that their union could be dissolved; it was declared that they had never in fact been joined in true marriage. Therefore, this phrase does not do against the indissolubility of marriage, but rather reaffirms it.
On the basis of Jesus' teaching and guided by the Holy Spirit, the Church has ruled that in the specially grave case of adultery it is permissible for a married couple to separate, but without the marriage bond being dissolved; therefore, neither party may contract a new marriage.
The indissolubility of marriage was unhesitatingly taught by the Church from the very beginning; she demanded practical and legal recognition of this doctrine, expounded with full authority by Jesus (Matthew 19:3-9; Mark 10:1-12; Luke 16:18) and by the Apostles (1 Corinthians 6:16; 7:10-11; 39; Romans 7:2-3; Ephesians 5:31).
The Greek word used here appears to refer to marriages which were not legally marriages, because they were within the forbidden degrees of consanguinity (Leviticus 18:6-16) or contracted with a Gentile. The phrase "except on the ground of unchastity" does not occur in the parallel passage in Luke 16:18. See also Matthew 19:9 (Mark 10:11-12), and especially 1 Corinthians 7:10-11, which shows that the prohibition is unconditional.Thus, porneia does not allow an exception to the indissolubility of marriage, and Mt 5:31-32 means that man and woman can separate and marry again only when their first marriage was never valid in the first place. This has been the constant tradition of the Church (see here). For more on divorce, go here and here.