The crowd heard Jesus' teaching about divorce: it clearly stirred the disciples too.  They asked Him about the implications of marriage breakdown (presumably including remarriage).  His answer did not depend on situational ethics but creation design.  Jesus said that remarriage after divorce is adultery.  He adds to this in Matthew 19:9: "I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery."  And to make the point equal for both parties, Jesus said this applied to a woman who divorces her husband too.  Although that was not allowed by Jewish law, it was permitted by the Roman occupation authorities.

For the disciples, this was not an academic point.  John the Baptist had denounced King Herod and his second wife Herodias for doing just that.  In brief, the back-story was that the king went to Rome to meet the Emperor Tiberius, and whilst there visited his half-brother Philip and his wife Herodias with whom he had a passionate affair.  Herodias then schemed that Herod should divorce his wife Phasaelis, and she would divorce Philip - so that they could legally marry.  John told Herod that it was not lawful before God for him to marry his brother's wife Mark 6:17-19).  Herodias was furiously angry about John's intervention and nursed a grudge against him.  That grew into a venomous hatred – leading to John's execution (Mark 6:21-29).

So, if the disciples were to agree with Jesus' teaching and preach it, what might happen to them?  When they asked Jesus to explain, He gave exactly the same response as John the Baptist.  Jesus was not just feeding the embryo apostles with impossible regulations for the future church.  He was setting God's standards into their hearts - standards based on what God had already done in creation and spoken in blessing over marriage.  The words, "… a man leaves his father and mother and is united to his wife, and they become one flesh" (Genesis 2:24), are repeated by Jesus (Matthew 19:5; Mark 10:7).  They are also used by the Apostle Paul as the basis for marital fidelity in Ephesians 5:31 because the marriage bond is the closest human representation of the commitment of Christ to His church.  It is therefore a significant Biblical doctrine, and remains a challenge for Biblical preachers today.  

Indeed, every part of real church life is based on what God has done – His creation, incarnation, atonement, ascension and Holy Spirit filling ... and what He has said He will do - Second Coming, judgement, heaven and hell, the new heavens and new earth.  So why do we dilute the practical teaching of Jesus?  When we replace the work of God by our own bright (and even passionate) ideas, the building cannot stand (Matthew 7:26-27).  Grace certainly exists to cover our past but was never intended to facilitate ungodliness in our future (Romans 6:1-4).  Grace squandered is as foolish as throwing gold into the sea.