As Matthew writes for a Jewish audience sensitive to the pressures (or, for many, requirement) to divorce the sexually unfaithful, an absolute prohibition of divorce would have been a stumbling block. Matthew's Jesus clarifies that the prohibition did not keep the husbands from divorcing adulterous wives, as expected in their legal traditions. Divorce would be permissible in these circumstances.Remarriage in Early Christianity, 155
Jesus's logic is that if the woman was divorced because of πορνεία, then she would already be an adulteress (and no one should marry an adulteress). If she was divorced for some other reason, the divorce was unlawful (since πορνεία is the only legitimate reason), and the woman is not free to marry another.Remarriage in Early Christianity, 153
Out of necessity, the innocent wife is made (i.e., caused to become) an adulteress by marrying another man. In other words, God's will for the original marital union renders any subsequent union adulterous, despite the legality of the divorce, the declared freedom to remarry in the divorce certificate, and the divorced wife's innocence.Remarriage in Early Christianity, 149