The exception phrase (Mt. 5:32; 19:9) applies to a permission to divorce and to remarry, contrary to the otherwise careful arguments of scholars like William A. Heth and Gordon J. Wenham in Jesus and Divorce.
Heth and Wenham argue for a permission to divorce but no remarriage and base their conclusions on two main arguments.
First, for five centuries (excepting for Ambrosiaster in the fourth) the church Fathers, and later the western Church up to the 16th century, permitted divorce but no remarriage.
Second, the astonishment of the disciples, after hearing Jesus on divorce, is best explained by a total ban on remarriage. Stott’s critique is helpful and we quote him fully.
“Although this case is strong, it is not conclusive. First, the early church Fathers could have been mistaken in this matter as they were in others. Secondly, the statement in Matthew 5.32 that a husband who illegitimately divorces his wife ‘causes her to commit adultery’ can be true only if after the divorce she remarries. Thirdly, the disciples’ astonishment leading to the teaching on celibacy could have had another cause. Their perception must certainly have been of the strictness of Jesus. Not only did he reject the trivial laxity of the Hillel school, but also Shammai’s interpretation, and indeed Moses’ own reference to ‘something indecent’, as being too imprecise. Only sexual infidelity could be admitted as a ground for breaking the marriage bond. This had been clearly recognized in the Old Testament because it was punishable by death. But the death sentence for adultery had fallen into desuetude [disuse], and in any case, the Romans did not permit the Jews to administer it. So when Joseph suspected Mary of unfaithfulness, he thought of divorce, not death (Matthew 1.18ff)…It seems then that [Jesus] abrogated the death penalty for sexual infidelity, and made this the only legitimate ground for dissolving the marriage bond, by divorce not death, and then only as a permission.” ( In his Decisive Issues Facing Christians Today, p. 295)
The early Church Fathers had very strange views on marriage and reveal tendencies against marriage. Jesus’ exception phrase is identical to that of Shammai and both permitted divorce on the ground of sexual immorality while dealing with Deut. 24 (logou porneias = davar ervah, in structure and concept). Shammai also held, like all Jews, that other grounds for divorce arose from breaches of Ex. 21.10-11.