I mounted the Biblical evidence that Jesus’ “exception” clause cannot mean that Jesus is “allowing” divorce in certain cases. That is not a tenable reading. If it were, then if you were divorced you could get remarried. But Jesus proscribes anyone from getting remarried, even if he is divorced.
So what does he mean by this “exception” clause? Here, theological speculation must come into play. A thing to observe is that the Pharisees ask Jesus whether there be any cause on account of which one may divorce one’s wife (See Cornelius a Lapide’s absolutely masterful commentary. If you do not own it, buy it. It is better than anything that has been written on the Gospels since. His is the way to read the Scriptures. Are there new methods and knowledge that can help? Yes. But his is the substance, the bulk of what is important for salvation.) The implicitly ask, also, whether one may remarry. Thus, Jesus has two questions on his hand. I have already given his answer to the second question: No, no validly married person may remarry. Period. Thus, Jesus puts an end to polygamy and “divorce” when by “divorce” one means dissolution of the bond. However, another question remains. Is there no cause on account of which there might be a separation? Although the bond be not dissolved, is there a cause whereby the matrimonial rights of one party can be forfeited vis-a-vis the other party?
Following this question, we discover One Possible Reading of the text. Perhaps Jesus uses the term “divorce” loosely, to mean “separation”. We have seen that in certain grave circumstances separation is allowable by Tradition. Today, our eyes usually only see the problem of physical abuse, a man beating a woman so unjustly as to give her the right to separate from him until such time as the cause for separation has been remedied. But there is also the problem of infidelity. The duty of Christian charity must reign supreme, in all cases. Thus, if there has been an infidelity, the noblest act, that which will secure salvation the more and true happiness and healing the more, is forgiveness. However, if the offended spouse has not approved the infidelity, tacitly or expressly, then she or he has the right to separate from bed and board. In all cases of separation, the spouse seeking separation must seek approval from the competent authority (check the Bishop’s office for whom to speak with. See Canon Law, Canon 1151ff.) Note that separation is not divorce; it is simply physical and financial and emotional distance. The separated couple is a married couple. Well, St. Paul takes our Lord’s command in this way. “To the married I give charge, not I but the Lord, that the wife should not separate from her husband (but if she does, let her remain single or else by reconciled to her husband)—and that the husband should not divorce his wife.” The double commandment (separation permissible, divorce prohibited) applies to each, to male and female. (This is a typical biblical shorthand.) Many of the Church Fathers read the commandment of Jesus as St. Paul reads it.
There remains A Second Possible Reading of the exception clause. By “divorce” in the “exception” case, Jesus indicates something like the annulment declaration. In a declaration of nullity, the proper Church official declares that there never was a bond in the first place. That the couple never was validly married. How does the exception of “unchasitity” fit into this annulment reading? The Greek term is porneia. (Where we get “pornography” from.) Porneia can refer to a variety of kinds of sexual deviance. It can refer also to incest (see, for instance, 1 Cor 5:1-2). In those days, many gentiles would marry close relations (cousins, half siblings, etc.). When the relation is close enough, it is incest. The Church never recognized such marriages (see Acts 15:20, 29). The Mosaic Law also forbad it (Lev 18:6-18). Some theologians speculate that Jesus is indicating a kind of situation in which supposedly married persons are not in fact married. In such cases, a true declaration of nullity would be possible (and necessary).
In fact, both of these readings are possible. Our Lord was not a mere “Marginal Jew,” as some falsely label him. Our Lord was an astoundingly brilliant teacher. A true master. A rabbi. He could say several things with one word. Thus, two distinct literal readings are indeed possible. For the Word of God is living and active; it is a two-edged sword.