At the very beginning of His earthly ministry, Jesus said that He had come to bring the Torah to fullness, not to destroy it. One aspect of this process was evidently the logical completion of the meanings placed in it when it first appeared. This is how Jesus also views marriage. The Torah, of course, knows the depth that unites spouses, binding them not only at the physiological or psychological level but also at the spiritual one. Yet, condescending to the weakness of fallen human nature, it permits divorce in certain cases. At first sight this is an absurd situation, for it concerns bonds that are not subject to human control at all, something with which one cannot play and which one cannot manipulate. But the fallen world is full of absurdity in general, and the Torah, making allowance for human nature, had to take this into account.
Jesus, however, does not wish to make allowance for fallen human nature: He has brought into the world the Kingdom where the complete transformation of this nature is possible, and His task is to prepare everyone who desires it to enter the Kingdom. That is why He speaks of the inadmissibility of divorce. With one exception: if one of the spouses is guilty of sexual immorality, something by definition incompatible both with marriage and with the Kingdom.
The spiritual standard is, of course, set extremely high here. The disciples understand this, which is why they say that it is then better not to marry. On the lips of a Jew, for whom family life was effectively a religious duty, such a statement sounded strange, to say the least. But Jesus, of course, understood everything perfectly and answered directly that in principle a situation is possible in which a person postpones family life until the triumph of the Kingdom, postponing it in order to build it no longer according to the laws of the untransformed world, but according to the laws of the Kingdom. Yet, in His own words, by no means everyone is capable of this: it must be given to a person, and it depends on that person no more than being born a eunuch or being made one by others.
Obviously, such a renunciation of family life cannot and should not be a universal principle. Jesus evidently looks at everything through the eyes of the Kingdom. At the same time, He looks at everything through the Torah, which Paul will later compare, not by chance, to a tutor who leads seekers to Christ.
