I agree with you, Rev. Winzer
@MW (which gives me great joy because even though I have disagreed with you in the past, I have always held your thoughts overall in high esteem).
I agree that there has already been a solid case made, and while I cannot add to it, perhaps a re-framing of the case laid out might help here.
To those struggling with remarriage after desertion: Consider the audiences of each text.
In Matthew, Christ is speaking to the Jews in Palestine who are law-abiding (in self-proclaimed principle, if not fully so in fact).
These are marriages that are formed under Torah and seek to follow Torah (again, in self-proclaimed principle at least). For Christ to say to these, the exclusive cause for divorce is the violation of "one-flesh" law in the form of adultery is not the same as Christ saying to all everywhere that all violations of "one-flesh" law is exclusively physical adultery.
One needs also to recall the heavy link between physical adultery and spiritual idolatry and apostasy that permeates the Scriptures.
I suspect it is that basis where Paul hears of the case of a spouse (or spouses) who believes the gospel, repents and follows Christ while married to one who does not repent, therefore coming upon a very hard time:
"Is not my unbelieving spouse a spiritual adulterer - if not a physical adulterer?" the distressed believing spouse cries in both loving concern and fear of a holy God and poised to separate heading towards a divorcement based on Matthew 19.
Ah, but wait, dear child of God! the Holy Spirit comes calming and giving balm through the pen of Paul and says "Peace be still".
This arrangement of one believing spouse married to a non-believing spouse does
not count as a violation of the "one-flesh" principle Jesus taught in Matthew 19 - not because there is no spiritual adultery present (even if no physical) but that the Spirit indwelling within the believing spouse sanctifies the entire household - even the unbelieving spouse within its walls.
Inspired by the Holy Spirit, Peter would later add in 1 Peter 3:1-6:
"Wives, in the same way submit yourselves to your own husbands so that, if
any of them do not believe the word, they may be won over without words by the behavior of their wives, 2;when they see the purity and reverence of your lives. 3;Your beauty should not come from outward adornment, such as elaborate hairstyles and the wearing of gold jewelry or fine clothes. 4;Rather, it should be that of your inner self,
the unfading beauty of a gentle and quiet spirit, which is of great worth in God’s sight. 5;For this is the way the holy women of the past who put their hope in God used to adorn themselves. They submitted themselves to their own husbands, 6;like Sarah, who obeyed Abraham and called him her lord. You are her daughters
if you do what is right and do not give way to fear." (Bolded added for emphasis)
From 1 Cor. 7 it seems true that if any believing spouse left a non-believing spouse because
they tried with words and the other would not hear, they would be guilty of sin, (we are all agreed here).
But if it is true that the believing spouse - walking in the same Spirit that raised Christ from the dead - stays with the non-believing spouse but the other rejects the believer and runs from the sanctifying Spirit that permeates the household, the deserting spouse is not necessarily innocent of adultery even if he or she
is innocent of the specific charge of physically sleeping with someone not his spouse.
Christ - through Paul and Peter - affirms this constitutes a violation of the "one-flesh" principle and the believing spouse is free to remarry ("not bound in such circumstances") and God will give peace to the scorned believer for the non-believing spouse did
not - in fact - abandon
her (or him), but they have indeed rejected the God of peace that is within the house.