But I say to you, whoever divorces his wife (except for porneia) causes her to commit adultery, and whoever marries a divorced woman commits adultery.
The Greek word porneia can have several possible meanings. It is a general word used to describe all sorts of sexual immorality, from prostitution to fornication to adultery to incest. In this context it is believed that porneia refers to relationships that are related so closely as to be considered incestuous.
Porneia is used in this context in 1 Cor 5:1 as well:
It is widely reported that there is immorality among you, and immorality of a kind not found even among pagans—a man living with his father’s wife.
Some commentators have thought that the word refers to adultery, but if the author was looking to make adultery the singular “exception” then he would have used the word moicheueō. The other problem with understanding the word as adultery is that neither Mark nor Luke include this so-called exception clause. Adultery is a problem that cuts across all cultural lines, its a moral issue that every group faces. It would seem illogical that Luke and Mark would have excluded this simple and normal issue from their sayings of Jesus.
It is also important to note that in Acts 15:28-29 when the Council of Jerusalem declares what Gentiles are to avoid, the list follows Leviticus exactly:
It is the decision of the holy Spirit and of us not to place on you any burden beyond these necessities, namely, to abstain from meat sacrificed to idols, from blood, from meats of strangled animals, and from unlawful marriage porneia]. If you keep free of these, you will be doing what is right. Farewell.’”
Idol Sacrifices Lev. 17:8-9
Blood Lev. 17:10-12
Things Strangled Lev. 17:13-14
*Porneia *Lev. 18:6-18
Leviticus 18:6-18 deals pretty much with forbidding incestuous relationships.
Jesus is speaking of divorce in its broadest sense. Jesus is acknowledging that some people might enter into marriages which are unlawful or inherently immoral. In such cases Jesus is saying that because the marriage was never a true marriage to begin with that divorce is not an issue.