Having read the first two unproductive pages of this thread, I turn our attention back to presuppositions of the OP. Our Secret friend chose to quote without attribution the Gospel of Matthew, Ch. 19, but the more complete version at Mark 10 would seem more appropriate as the longer more original version further authenticated by the usual presumption (which I don't personally accept) that Mark was used by Matthew (they actually used a common source).
Since I got started inserted the verses/words I took incorrectly that Secret had omitted, here we go, using the same New International Version he employed, but from Mark (yet leaving Matthean wording Stephan already has the same phrase--with brackets denoted my insertions and ellises my deletions where Mt by usual scholarly standards added to Mark:
Secret Alias wrote:
The obvious starting point is ANY discussion of the Pentateuch which obviously puts Jesus as the spokesperson for the mind of the author of the gospel (= Paul, Mark, take your pick of names):
[Mark] 10:2] Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife {for any and every reason?}”
[3 "What did Moses command you?" he replied.
They said, "Moses permitted a man to write a certificate of divorce and send her away."]
{4} [6]“Haven’t you read,” he replied, [6]“that at the beginning the Creator ‘made them male and female,’ {5} [7]and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? {6} So they are no longer two, but one {flesh}. [9]Therefore what God has joined together, let no one separate.”
[10] When they were in the house again, the disciples asked Jesus about this.]
{7 “Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?”}
{8 Jesus replied, “{Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning.} {9 } I tell you that [11] anyone who divorces his wife,{ except for sexual immorality}, and marries another woman commits adultery [against her].”[12 And if she divorces her husband and marries another man, she commits adultery."]
{10 The disciples said to him, “If this is the situation between a husband and wife, it is better not to marry.”
11 Jesus replied, “Not everyone can accept this word, but only those to whom it has been given. 12 For there are eunuchs who were born that way, and there are eunuchs who have been made eunuchs by others—and there are those who choose to live like eunuchs for the sake of the kingdom of heaven. The one who can accept this should accept it.”}
........
Skipping his commentary following, more important is to recognize that Higher Criticism affirms the superiority of what can be multiply attested. Yes, skeptics of Lower Criticism attack textual development during the initial centuries of Christianity, but it would take an extreme I'm unaware of to reject a text stated equivalently in more or more gospels. Best attestation in practice is verses in Luke that are also in Mark or Matthew. In this OP we have both Mark and Matthew, which to me is excellent wherever they agree, because here as in most of these first two gospels they are based on a common source, whether the Proto-Gospel, Proto-Matthew, or some other hypothetical text at a particular stage of development (being my hypothesis). OK, clearly above we have some solid stuff by my analysis. We even know from the Gospel of John (quadruple attestation) that Jesus argued with the Pharisees, the first verse above. We don't know if the next verse was primitive, as it does not appear in Matthew and is not included by Stephan--maybe the Evangelist just didn't want to give the enemy equal time? In both we find Jesus's rationale. Does anyone really question that Jesus was a rather clever fellow? That's the whole raison d'etre of the Jesus Seminar, to prove that Jesus was their precursor as the wise-cracking Cynic philosopher. Sure, lots of scholars in contrast don't like Jesus, but putting him done as a megalomaniac or manic-depressive does not lessen the probability that he stood his ground against the authorities and maybe backed them into corners. That's a 21st Century Forensic Psychiatry lesson for the less informed here. So Matthew 19:4-6 stands tall. I would likewise argue that Mt. 19:7 captures the original better than the generic Mark 10:10. Matthew 19:9 equals Mark 10:11, so Jesus really did forbid divorce against rabbinic leniency (toward men!). Notice that Mt. 19:9 is NOT followed by Mark 10:12 that grants equal rights (wrongs, in this case) to women. Perhaps Matthew suppressed this verse because it was irrelevant as Jewish law (in contrast to Roman law) did not permit women to divorce their husbands (or could not conceive that a woman would, as she would be sentencing herself to poverty or prostitution?).
All told, whatever stands agreed above really does go back to Jesus, as does much of the material Matthew or Mark omitted for whatever reason.