“Whoever divorces his wife, except for unchastity [porneia] . . . and marries another, commits adultery.”
1.The meaning of the [Greek] term [porneia] is uncertain. It designates illicit sexual behavior, which may include adultery. We cannot jump to the conclusion that the term [porneia] means adultery, because Greek has a specific word for that . . .
2. According to the classic Catholic solution, Matthew is not presenting a real exception because the verb [Greek for divorces] does not refer to divorce in the sense of the dissolution of the marriage that would clear the way for a second marriage. The aforementioned verb refers instead to a separation from bed and the cessation of cohabitation without a second marriage in the case of an adulterous wife. In this interpretation, the clause “except for unchastity” would have to do with the separation from bed and would imply that this is lawful only in the case of a woman guilty of adultery. . . .
3. It is most likely that [porneia] here is the translation of the Hebrew term zênŭt, understood as an incestuous union with forbidden degrees of relationship (cf.Lev 18:6-18). In such a case there is in fact no marriage and a decree of nullity would be required rather than a divorce. – from “Can Divorced and Civilly Remarried Persons Receive Communion?” in Eleven Cardinals Speak