Today's teaching is in two parts. Firstly, there is Jesus' response to a Pharisee's question and secondly a response to a question of the disciples. Let us look at them separately.
I repeat again that we must make a distinction about the questioning of Jesus. Some questions re sincere and often others just challenge his authority. Today's question is of the latter variety. It give Jesus a choice - to affirm the Law or to allow an exception. It is a trap. Let us look at it.
Jesus notes that Moses allowed divorce because they were so unteachable.
Jesus reaffirms in Mark without exception that "from the beginning God made them male and female... They are no longer two, therefore, but one body. So then, what God has united, man must not divide."
Jesus uses two authorities in giving his answer. He quotes Moses and he quotes the Law. That is a rabbinical argument. The argument is over.
The second part of the discussion is interesting. "Back in the house the disciples questioned him again about all this, and he said to them, 'The man who divorces his wife and marries another is guilty of adultery too.'" Jesus reaffirms his belief in God's original intention that marriage is a life-long commitment. Other New Testament writers are different (e.g. Mt 5:32, 1Cor 7.10-16). Mark doesn't make any exceptions.
It is noted that Jesus places husband and wife on equal footing.
Divorce has always been a difficult pastoral issue from at least the time of Moses. As Catholics we are lucky to have a Church that leads us through the difficult times. the Marriage tribunals in every diocese are staffed by extremely pastoral people who are available to help thousands of people through difficult times. In modern times there have been great advances as the church tackles issues on divorce and remarriage and communion, not to mention bio-ethical issues round the creation of human life. Never be afraid to seek pastoral advice.