The heart of all Church teaching regarding the Sacrament of Marriage can be found in Jesus' own words in the Gospel as He draws us to find its origin in the Genesis Creation account and to see that it is an indissoluble bond between man and woman.
Matthew 19: 1-12
"Now when Jesus had finished these sayings, he went away from Galilee and entered the region of Judea beyond the Jordan; and large crowds followed him, and he healed them there.
And Pharisees came up to him and tested him by asking, “Is it lawful to divorce one’s wife for any cause?” He answered, “Have you not read that he who made them from the beginning 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’? So they are no longer two but one. What therefore God has joined together, let not man put asunder.” They said to him, “Why then did Moses command one to give a certificate of divorce, and to put her away?” He said to them, “For your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. And I say to you: whoever divorces his wife, except for unchastity, and marries another, commits adultery; and he who marries a divorced woman, commits adultery."
The disciples said to him, “If such is the case of a man with his wife, it is not expedient to marry.” But he said to them, “Not all men can receive this precept, but only those to whom it is given. For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. He who is able to receive this, let him receive it.”
Age: the Church, based on reasons of physical and intellectual maturity, requires that men be at least 16 years of age and women at least 14 years of age.
Physical impedement: sexual intercourse is required to consummate a sacramental marriage between the two spouses. If a person is physically unable in this regard it is not possible for the sacrament to fully come into existence. However, the physical inability to have children (sterility) is NOT an impedement to marriage!
Disparity of Cult: if a Catholic wishes to marry a non-Catholic in a sacramental union the local ordinary (bishop) must give a dispensation to the couple prior to the wedding day.
Previous Existing Marriage: one cannot be already in an existing marriage and contract a new marriage. This refers not only to previous marriages in the Church but extends also to any valid, natural (civil) marriage which has been contracted in the past.
Sacred Orders: those who have been previously ordained to Holy Orders (priesthood) cannot enter into the sacrament of marriage.
Vow of Chastity: those who have taken vows of chastity within the Church such as priests or consecrated religious cannot enter the sacrament of marriage unless they have been properly dispensed by Church authorities.
Crime: grave acts such as kidnapping of a spouse or the killing of a previous spouse so as to enter into a new marriage render it impossible for a person to enter into the sacrament of marriage.
Consanguinity, Affinity, Adoption, and Spiritual Relationship: those who are connected to one another in a family by degrees of blood relations, adoption, or other bonds, as well as those connected spiritually (godparents-godchildren and confirmation sponsors-confirmandi for example) cannot enter into the sacrament of marriage with one another.