An excellent interview by the Word on Fire website with H.E. Thomas Cardinal Collins:
Brandon Vogt: Much of the current discussion about divorce, remarriage, and communion is clouded by confusion. What does the Church actually teach on these issues and why?
Cardinal Thomas Collins: The Catholic Church simply teaches what Jesus teaches: marriage is an unbreakable covenant between a man and a woman, faithful in love and open to the gift of life. Divorce and remarriage is not allowed when it is a matter of a valid, sacramental, and consummated marriage.
When Jesus was preaching in Galilee, divorce and remarriage was accepted in society. The law of Moses allowed for it (Deuteronomy 24:1-4). The teaching of Jesus that divorce and remarriage is not allowed was revolutionary. It was even an indication of his claim to divinity, for only God has the authority to over-rule the law of Moses. Jesus went back to creation itself for the foundation of the unbreakable bond of marriage between a man and a woman: "Have you not read that from the beginning the Creator 'made them male and female' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh.' So they are no longer two, but one flesh. Therefore, what God has joined together, no human being must separate" (Matthew 19: 1-12). In light of current controversies, it is also helpful to note that Jesus also asserts plainly something that until recently was obvious to everyone: marriage is between a man and a woman.
.... the full interview may be read here.