In the beginning, God instituted marriage as a permanent arrangement, in Matt 19:6-9 Jesus explains this to us. However in the last verse, Matt 19:9, he puts a qualification on it. Does this mean that if our spouse is unfaithful we are free to divorce and remarry? The word he uses porneia is often translated as unfaithfulness. Is this what it means?
In the beginning of Matt 19, the Jews seem to be asking for Jesus' opinion in a rabbinic conflict in interpreting Deut 24:1. The school of Shammai allowed divorce only for adultery, the school of Hillel allowed divorce tor any reason. They are asking if Jesus favors Hillel.
Jesus' answer, that divorce was not intended from the beginning, seems to deny any possibility of divorce, it is as if Deut 24:1 did not exist. This prompts the Jews to ask the logical question, why did Moses allow divorce if God did not want it? In other words, why does the Bible contain this verse if we cannot divorce?
Jesus now tells them that divorce was an allowance made for the hard hearts of Moses' people. Deut 24:1 does not contain God's plan for marriage as it was from the beginning, and Jesus is now restoring that original plan. It is at this point that he introduces the clause "except for porneia."
If porneia is to be translated as a synonym for adultery (unfaithfulness) then Jesus has decided in favor of the interpretation of Shammai, and not restored the meaning of marriage. Furthermore, if he intended porneia to mean adultery, his initial response to the Jews was unnecessarily confusing. Why talk about the way marriage was intended and proclaim divorce an allowance made for hard hearts under the old law if he simply agreed with the school of Shammai? Finally, porneia has a less precise meaning than moichao, the word for adultery. Thus, we have good reason to suspect that "marital unfaithfulness" is not a good translation.
The normal or basic meaning of "porneia" is sexual immorality. It also can be a pejorative way of referring to either concubinage, or some marriage that is not legal on other grounds. This usage is well established in rabbinic Judaism of Jesus' day. Even the Bible supports this usage of "porneia" as meaning an "illegal marriage" in 1 Cor 5:1, and Acts 15:20 (where it is thought to mean a marriage contrary to Jewish law). Divorcing a concubine and remarrying is not adultery because the concubinage is not a marriage but porneia, sexual immorality. One might well ask, however, why should he bring concubinage up at all? My answer is this: If Jesus intends for his listeners to put his new teaching into practice, he should give them the most relevant qualification. Is a marriage determined by contract, or by becoming one body with a woman (1 Cor 6:16)?
If this is so, it is also worthy of note that by referring to a concubinage (or any non-legal sexual relationship) as porneia, he is really making the Christian law of sexuality more difficult in two ways: by allowing no remarriage and disallowing concubinage. Is it any wonder that the apostles conclude that it is better not to marry? (Mat 19:10)