[The RSVCE carries a note which reads: “unchastity”: 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.] **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. herefore, this phrase does not do against the indissolubility of marriage, but rather reaffirms it.