Some scholars point out that the original Greek of Matt.19.9 probably didn't say "except" but said "not" or "excluding", so that there is no exception for adultery in Matt.19. What version of the Greek text are you relying on?
I have used the more common Greek version in my study, but the conclusions of my work aren’t any different when the other reading is used. However, the question is interesting.
There have been some very useful studies by Allen Guenther, Paul Dixon and Leslie McFall. The second two point out that Erasmus was probably wrong when he used the text "ei mé" ('except') in Mt.19.9. The original Greek was probably simply "mé" ('not'). Allen points out that even if the text was "ei mé", the normal way to translate this is “excluding” not “excepting”. This leads them to conclude that Jesus did not allow divorce for adultery in Matt.19.
BTW, I welcome scholarship which looks deeply into these issues, whatever conclusions it comes to. Leslie is a good friend of mine, and Allen’s article is published in a journal of which I am an editor. Every avenue needs to be explored in scholarship, and no stone unturned in our search for the eternal truths of the Bible.
They have pointed out a very important fact – that our early Bibles, including the King James version, relied on a Greek text which may be defective at Matt.19.9. It is likely that the original text said (literally)
“whoever divorces his wife not (mé) [for] immorality (porneia) and marries another, commits adultery”
whereas most translations are based on a text which says:
“whoever divorces his wife except (ei mé) for immorality (porneia) and marries another, commits adultery”
The meaning of the simple mé without the ei is rather unclear, but the most likely meaning is “excluding”. Allen pointed out that even if the text reads ei mé it can also be translated as “excluding”. So the question comes down to: does Matt.19.9 mean “excluding” or “excepting”.
So why do almost all English Bibles translate it as "except"? Partly because it makes better sense, but mainly because Matthew makes clear that this is the meaning in the parallel at Mt.5.32:
The two verses are very similar, but with several small differences, as seen when we translate them over-literally to highlight these differences:
Matt. 5.32: But I say to you that everyone divorcing his wife except (parektos) for a cause of immorality makes her commit adultery.
Matt. 19.9: And I say to you that whoever has divorced his wife if not (ei mé) for immorality and marries another commits adultery.
The two versions are clearly saying the same thing - that if you get remarried after the type of divorce Jesus is talking about, you are committing adultery. In other words, the divorce is invalid, so you are committing adultery because you are still married. In Matt.19.9 he points out this consequence applies to the man who does this divorcing, and in Matt.5 he points out it also applies to his wife. Jesus presumably applied it to both when he spoke on the subject, and also said much more, but Matthew had to summarise it.
Jesus didn’t speak in Greek, so Matthew is translating Jesus' words, and his translation is different in these two versions at almost every phrase.
* Matt.5 starts with ego de logo humin 'I however say to you' while Matt.19 has logo de humin 'And I say to you'
* Matt.5 uses pas 'everyone', and Matt.19 uses hos an 'whoever'.
* Matt.5 uses apoluon 'divorcing', and Matt.19 uses apolusé 'may have divorced'.
* Matt.5 he has 'logou porneias 'a cause of immorality’ and Matt.19 has simply porneia 'immorality'.
* Matt.5 applies this to the wife ('causes her to commit adultery') and Matt.19 applies it to the man who did the divorcing ('he commits adultery')
* Matt.5 uses parektos 'except' (Guenther's article shows this is clearly the meaning) and Matt.19 uses ei mé
The fact that he has slightly different Greek for 'except' in both passages does not surprise me. In fact, given all the other variations, it would be surprising if he had translated it exactly the same way in both. Even the earliest church commentators understood this to mean "except". Origen (2nd C) already did so, and even added ei to make it absolutely clear that it means “except”: "He hinders any one putting away a wife, unless (ei mé) she be caught in fornication.... (Origen, Commentary on Matthew 14.24.44, ANF)
This leaves us with a question. Why didn't Matthew use ei mé epi which would have been unambiguous? I don't know for sure, but I suspect it was because he was trying to translate Jesus' words exactly. The Aramaic or Rabbinic Hebrew for 'except' would be 'ala' im which is literally 'not if' - and the Greek ei mé is literally 'if not'.
But it didn't really matter whether Matthew left out the ei or not - everyone would have understood him. The context makes the meaning very clear. The Pharisees have asked Jesus about a particular type of divorce which they were currently debating about - the "Any Cause" divorce. Jesus condemned the Any Cause divorce which was invented by the Hillelite Pharisees to allow divorce for any little thing. The Hillelites had created their new 'Any Cause' divorce by dividing the phrase “a cause of immorality” (Dt.24.1) into two ground for divorce – ‘immorality’ and 'a cause'. Jesus thought that this denied the plain meaning of the phrase which was only talking about one type of divorce. Jesus happened to agree with the Shammaites on this occasion, though on other matters Jesus disagreed with the Shammaites – when there are only two points of view, you have to agree with someone! The Shammaites had pointed out that this whole phrase refers to one type of divorce - ie it refers to 'nothing except immorality'. Jesus quoted their slogan to show his agreement and concluded that anyone who used the ‘Any Cause’ type of divorce wasn't properly divorced - so if they remarried they were committing adultery.
In the end, it doesn’t really matter whether you translate this as “excepting” or “excluding”. It comes to the same thing.
If you ignore the fact that “Any Cause” was a type of 1st century divorce which Jews were debating at the time of Jesus, you would understand Jesus’ conclusion as:
"Whoever divorces his wife [for any cause], excepting/excluding immorality, then if they remarry they are committing adultery [because their divorce is invalid]
If you emphasise the mé as a simple “not” instead of “excepting”, then you could understand Jesus’ conclusion as:
"Whoever divorces his wife [for any cause], not [even for] immorality, then if they remarry they are committing adultery [because their divorce is invalid]
If you take into account the debate about the “Any Cause” divorce which was derived from Deut.24.1, you would understand Jesus’ conclusion as:
"Whoever divorces his wife [for ‘Any Cause’, which isn’t in Deut.24.1], unless this is for immorality [which is in Deut.24.1], then if they remarry they are committing adultery [because their divorce is invalid]”.
Of course Jesus would not be pedantic and include all the words in square brackets. His meaning was obvious to his listeners who knew all about the new popular divorce for ‘Any Cause’. For us, his message is more obscure until we stand in their shoes.