What is your source on this meaning of the word porneia? I am very curious.
We know that a legitimate/lawful marriage cannot ever be dissolved unless one of the spouses dies, and so therefore Jesus was talking about illegitimate/unlawful marriages according to God and not simply about adultery in general. Adultery by either of the spouses is not grounds to dissolve a marriage according to God's law. They may separate and get a civil divorce, but neither is allowed to remarry unless their spouse dies.
Matthew 19:6
So they are no longer two, but one flesh. Therefore, what God has joined together, no human being must separate. NABRE
Matthew 19:9
I say to you, whoever divorces his wife (unless the marriage is unlawful) and marries another commits adultery.” NABRE
Here are some writings from the early church fathers:
The Early Church Fathers on Divorce and Remarriage
The Early Church Fathers taught that a legitimate marriage lasted to death. And if separation became necessary a person was to remain single as long as the former spouse was alive. This was not a matter of Church law per-se but a direct result of the words of Jesus: “Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery” (Luke 16:18).
Hermas
What then shall the husband do, if the wife continue in this disposition [adultery]? Let him divorce her, and let the husband remain single. But if he divorces his wife and marries another, he too commits adultery (
Shepherd 4:1:6 [
A.D. 80]).
Justin Martyr
In regard to chastity, [Jesus] has this to say: If anyone look with lust at a woman, he has already before God committed adultery in his heart. "And, whoever marries a woman who has been divorced from another husband, commits adultery." According to our Teacher, just as they are sinners who contract a second marriage, even though it be in accord with human law, so also are they sinners who look with lustful desire at a woman. He repudiates not only one who actually commits adultery, but even one who wishes to do so; for not only our actions are manifest to God, but even our thoughts (
First Apology 15 [
A.D. 151]).
Clement of Alexandria
That Scripture counsels marriage, however, and never allows any release from the union is expressly contained in the law: "You shall not divorce a wife, except for reason of immorality." And it regards as adultery the marriage of a spouse, while the one from whom a separation was made is still alive. "Whoever takes a divorced woman as wife commits adultery," it says: "for if anyone divorces his wife, he debauches her"; that is, he compels her to commit adultery. And not only does he that divorces her become the cause of this, but also he that takes the woman and gives her the opportunity of sinning; for if he did not take her, she would return to her husband (
Miscellanies 2:23:145:3 [
A.D. 208]).
Origen
Just as a woman is an adulteress, even though she seems to be married to a man, while a former husband yet lives, so also the man who seems to marry her [and] who has been divorced does not marry her, but, according to the declaration of our Savior, he commits adultery with her (
Commentaries on Matthew 14:24 [
A.D. 248]).
Council of Elvira
Likewise, a woman of the faith [i.e., a baptized person] who has left an adulterous husband of the faith and marries another, her marrying in this manner is prohibited. If she has so married, she may not at any more receive communion — unless he that she has left has since departed from this world (canon 9 [
A.D. 324]).
If she whom a catechumen [an unbaptized person studying the faith] has left shall have married a husband, she is able to be admitted to the fountain of baptism. This shall also be observed in the instance where it is the woman who is the catechumen. But if a woman of the faithful is taken in marriage by a man who left an innocent wife, and if she knew that he had a wife whom he had left without cause, it is determined that Communion is not to be given to her even at death (canon 10).
Ambrose of Milan
No one is permitted to know a woman other than his wife. The marital right is given you for this reason: lest you fall into the snare and sin with a strange woman. "If you are bound to a wife, do not seek a divorce"; for you are not permitted, while your wife lives, to marry another (
Abraham 1:7:59 [
A.D. 387]).
You dismiss your wife, therefore, as if by right and without being charged with wrong doing and you suppose it is proper for you to do so because no human law forbids it; but divine law forbids it. Anyone who obeys men ought to stand in awe of God. Hear the law of the Lord, which even they who propose our laws must obey: "What God has joined together let no man put asunder" (
Commentary on Luke 8:5 [
A.D. 389]).
Jerome
Wherever there is fornication and a suspicion of fornication a wife is freely dismissed. Because it is always possible that someone may calumniate the innocent and, for the sake of a second joining in marriage, act in criminal fashion against the first, it is commanded that when the first wife is dismissed a second may not be taken while the first lives (
Commentaries on Matthew 3:19:9 [
A.D. 398]).
Pope Innocent I
[T]he practice is observed by all of regarding as an adulteress a woman who marries a second time while her husband yet lives, and permission to do penance is not granted her until one of them is dead (
Letters 2:13:15 [
A.D. 408]).
Augustine
Neither can it rightly be held that a husband who dismisses his wife because of fornication and marries another does not commit adultery. For there is also adultery on the part of those who, after the repudiation of their former wives because of fornication, marry others. This adultery, nevertheless, is certainly less serious than that of men who dismiss their wives for reasons other than fornication and take other wives. Therefore, when we say; "Whoever marries a woman dismissed by her husband for reason other than fornication commits adultery," undoubtedly we speak the truth. But we do not thereby acquit of this crime the man who marries a woman who was dismissed because of fornication. We do not doubt in the least that both are adulterers. We do indeed pronounce him an adulterer who dismissed his wife for cause other than fornication and marries another, nor do we thereby defend from the taint of this sin the man who dismissed his wife because of fornication and marries another. We recognize that both are adulterers, though the sin of one is graver than that of the other. No one is so unreasonable to say that a man who marries a woman whose husband has dismissed her because of fornication is not an adulterer, while maintaining that a man who marries a woman dismissed without the ground of fornication is an adulterer. Both of these men are guilty of adultery (
Adulterous Marriages 1:9:9 [
A.D. 419]).
From:
http://www.staycatholic.com/ecf_divorce_and_remarriage.htm