From: Mark 10:1-12
The Indissolubility of Marriage
 And He (Jesus) left there and went to the region of Judea and beyond the Jordan, and crowds gathered to Him again; and again, as His custom was, He taught them.
 And Pharisees came up and in order to test Him asked, "Is it lawful for a man to divorce his wife?"  He answered them, "What did Moses command you?"  They said, "Moses allowed a man to write a certificate of divorce, and to put her away."  But Jesus said to them, "For your hardness of heart he wrote this commandment.  But from the beginning of creation, 'God made them male and female.';  `For this reason a man shall leave his father and mother and be joined to his wife,  and the two shall become one.' So they are no longer two but one.  What therefore God has joined together, let not man put asunder."
 And in the house the disciples asked Him about this matter.  And He said to them, "Whoever divorces his wife and marries another, commits adultery against her; 12] and if she divorces her husband and marries another, she commits adultery."
1-12. This kind of scene occurs often in the Gospel. The malice of the Pharisees contrasts with the simplicity of the crowd, who listen attentively to Jesus' teaching. The Pharisees' question aimed at tricking Jesus into going against the Law of Moses. But Jesus Christ, Messiah and Son of God, has perfect understanding of that Law. Moses had permitted divorce because of the hardness of that ancient people: women had an ignominious position in those primitive tribes (they were regarded almost as animals or slaves); Moses, therefore, protected women's dignity against these abuses by devising the certificate of divorce; this was a real social advance. It was a document by which the husband repudiated his wife and she obtained freedom. Jesus restores to its original purity the dignity of man and woman in marriage, as instituted by God at the beginning of creation. "A man leaves his father and his mother and cleaves to his wife, and they become one flesh" (Genesis 2:24): in this way God established from the very beginning the unity and indissolubility of marriage. The Church's Magisterium, the only authorized interpreter of the Gospel and of the natural law, has constantly guarded and defended this teaching and has proclaimed it solemnly in countless documents (Council of Florence, "Pro Armeniis"; Council of Trent, "De Sacram. Matr."; Pius XI, "Casti Connubi"; Vatican II, "Gaudium Et Spes", 48; etc.).
Here is a good summary of this doctrine: "The indissolubility of marriage is not a caprice of the Church nor is it merely a positive ecclesiastical law. It is a precept of natural law, of divine law, and responds perfectly to our nature and to the supernatural order of grace" (St J. Escriva, "Conversations", 97). Cf. note on Matthew 5:31-32.
5-9. When a Christian realizes that this teaching applies to everyone at all times, he should not be afraid of people reacting against it: "It is a fundamental duty of the Church to reaffirm strongly [...] the doctrine of the indissolubility of marriage. To all those who, in our times, consider it too difficult, or indeed impossible, to be bound to one person for the whole of life, and to those caught up in a culture that rejects the indissolubility of marriage and openly mocks the commitment of spouses to fidelity, it is necessary to reaffirm the good news of the definitive nature of that conjugal love that has in Christ its foundation and strength (cf. Ephesians 5:25).
"Being rooted in the personal and total self-giving of the couple, and being required by the good of the children, the indissolubility of marriage finds its ultimate truth in the plan that God has manifested in His revelation: He wills and He communicates the indissolubility of marriage as a fruit, a sign and a requirement of the absolutely faithful love that God has for man and that the Lord Jesus has for the Church.
"Christ renews the first plan that the Creator inscribed in the hearts of man and woman, and in the celebration of the sacrament of matrimony offers `a new heart': thus the couples are not only able to overcome `hardness of heart' (Matthew 19:8), but also and above all they are able to share the full and definitive love of Christ, the new and eternal Covenant made flesh. Just as the Lord Jesus is the `faithful witness' (Revelation 3:14), the `yes' of the promises of God (cf. 2 Corinthians 1:20) and thus the supreme realization of the unconditional faithfulness with which God loves His people, so Christian couples are called to participate truly in the irrevocable indissolubility that binds Christ to the Church, His bride, loved by Him to the end (cf. John 13:1).
"To bear witness to the inestimable value of the indissolubility and fidelity of marriage is one of the most precious and most urgent tasks of Christian couples in our time" (John Paul II, "Familiaris Consortio", 20).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.