But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery (Mt 5:32).
And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery (Mt 19:9).
This raises several questions:
1. The Markan parallel has no exceptions. So that raises the question of which text preserves the original wording of Christ's statement. Several theories:
i) Matthew has liberalized the absolute prohibition of Jesus. That interpretation is incompatible with the inerrancy or historicity of Matthew.
ii) Matthew added that caveat to the original. But that's because Jesus and the disciples took that exception for granted. General statements often presume unstated qualifications. It's not uncommon in Scripture for formally unconditional statements to be implicitly conditional.
Although I think that's a legitimate explanation, it's not my preference (see below).
iii) It's quite possible or probable that Mark preserves the original wording of what Jesus said on that particular occasion, but afterwards, the disciples questioned Jesus to clarify the scope of his prohibition–at which point he qualified his initial statement. Matthew combines what Jesus said on two separate occasions: the initial statement and the follow-up.
I think that's a very realistic scenario. It's consistent with how Gospel writers edit material. It's consistent with the fact that his disciples did ask him follow-up questions. And it's consistent with the historicity and inerrancy of Matthew.
However, any explanation is speculative. Although that's an interesting question in its own right, for Christians who affirm the inspiration of the Gospels, we don't have to reconstruct the original exchange for Matthew's version to be authoritative.
2. Most commentators think this text supplies a justification for divorce. In other words, they think it refers to sexual sin within marriage. The standard candidate would be adultery.
It's striking how pious Catholics act as if it's unquestionable that Jesus forbids divorce, when on the face of it, Jesus seems to allow for divorce in this very text. There's a general prohibition against divorce, but here's an exception.
3. Because Catholicism teaches the indissolubility of marriage, it can't take the position that this refers to extramarital sex, so it it must refer to something else.
i) One alternative is premarital sex. Hence, that would be a grounds for annulment rather than divorce.
ii) I don't object in principle to the concept of annulment. I think there are cases where that's a valid principle. However, I object to a theory of annulment that's driven by false dogma regarding the indissolubility of marriage. Moreover, that results in a declaration of annulment in cases where that's not a valid application of the principle. In reality, the Catholic church annuls marriages for the same reasons people divorce. In practice, it's a distinction without a difference.
iii) One problem with the Catholic interpretation of Matthew is how we'd be able to determine from this text that Jesus is providing grounds for annulment rather than divorce. Indeed, what clue does the reader have that Jesus is talking about annulment rather than divorce?
The larger context concerns divorce. Jesus is countering a lax position on divorce. There's no indication that Jesus suddenly pivots to annulment.
We could turn the Catholic interpretation around. Although the Catholic position takes this to be referring to annulment, how can a reader distinguish grounds for annulment from grounds for divorce? What would Jesus say differently if he were talking about divorce?
iv) Here's another problem with the Catholic interpretation: this is recorded in a Christian gospel. The new covenant actively reaches out to Gentiles.
However, most pagan converts to Christianity would have sexual experience prior to marriage. So the Catholic interpretation invalidates just about every gentile Christian marriage, whether that was contracted before they became Christian or afterwards. That would be true for anyone who wasn't a virgin on his or her wedding night. When you consider that Christianity is a missionary religion, expanding into a world where premarital sex was the norm, the implication that all those married Christian converts are living in sin is extraordinary. By that logic, nearly every pagan convert to Christianity would have to desert their spouse and become celibate. After all, if premarital sex disqualifies a person from entering into a genuine marriage, then you cross a line of no return.
v) A further problem with the Catholic interpretation is that porneia has a broad semantic range. Although that includes premarital sex, the meaning of the word is scarcely confined to premarital sex. Moreover, in the context of a discussion about marriage, divorce, and remarriage, adultery is the default sin which the disciples, or Matthew's audience, would think of, since that's the sexual sin which typically precipitates divorce. Although the word means more than adultery, we wouldn't expect it to mean less than that–especially in this context.
So there's nothing in the context to single out premarital sex. To the contrary, the indicators suggest otherwise.
4. Another Catholic interpretation takes this to be a reference to incestuous marriage. An advantage of that alternative, from a Catholic standpoint, is that incestuous marriages are invalid, so that's consistent with annulment rather than divorce. There are, however, serious problems with that interpretation:
i) Once again, although incest would be covered by porneia, the word doesn't single out incest to the exclusion of other sexual sins. Moreover, the first kind of sexual sin which the disciples, and Matthew's audience, would naturally think of in a discussion about marriage, divorce, and remarriage, is adultery–since that's the leading cause of divorce.
ii) Due to the nearly universal incest taboo, even in Greco-Roman paganism, we wouldn't expect Jesus to be alluding to something that esoteric.
iii) There is, moreover, the question of whether Jesus would even classify an incestuous liaison as "marriage".