Don't inherently know "why" Jesus would say anything... but I do know that I would rather ask HIM why, than someone else. I can only tell you what I feel/believe. And I feel/believe that Jesus would prefer people to stay married at all costs, but when the cost is too great, then the decision is permanent. If you divorce someone, then you have cast the first stone, so to speak. And as I said, Jesus regarded looking on another with lust in the heart every bit as sinful as extra-marital sex in the flesh. If you can cut off someone's avenue to repentance and reconciliation, you stand in judgment of them. If you do not forgive others, your Heavenly Father will not forgive you.
An unfaithful spouse cannot be a believing spouse. It is a complete contradiction. Therefore, if the unbelieving spouse desires to dwell with you, don't seek to be loosed. But if you do leave, Christ commands you to remain single.
As for keeping marriage sacred: If anyone searches their heart, they will know that they could have been more loving, could have been more giving, could have taken better care of themself and their partner ... and so bears some responsibility for any condition which impinges on the marriage - including infidelity. Only in the case of undisclosed premarital situations can a person can be truly said to bear no fault. Everything that comes after marriage is committed by two joint-heirs of salavation, and we have responsibility for one another.
It is the undisclosed fornication that Jesus referred to when saying that such reasons for break-up did not result in adultery conditions. Because in the eyes of God, and man, there was no marriage which was legally or morally entered into. Every prospective spouse has a right to know who they are marrying, inside and out.
If the Spirit has not taught someone the first principles of walking in faith (forgiveness), and they need justification in the written word to do what you want to do they will find it. Jesus, however, did not "let people off", but said very clearly if you do not forgive all of your brother's (or wife's or husband's... or are you going to say I am speculating, that he said "brother" so that is where it ends?!) trespasses, your Heavenly Father will not forgive you.
