What is marriage? Let’s find out the two institutions of marriage. The Natural and the Supernatural.

A Natural Institution:

Marriage is a practice common to all cultures in all ages. It is, therefore, a natural institution, something common to all mankind. At its most basic level, marriage is a union between a man and a woman for the purpose of procreation and mutual support, or love. Each spouse in a marriage gives up some rights over his or her life in exchange for rights over the life of the other spouse.

While divorce has existed throughout history, it has been rare until recent centuries, which indicates that, even in its natural form, marriage is meant to be a lifelong, union.

The Elements of a Natural Marriage:

As Fr. John Hardon explains in his Pocket Catholic Dictionary, there are four elements common to natural marriage throughout history:

1. It is a union of opposite sexes.

2. It is a lifelong union, ending only with the death of one spouse.

3. It excludes a union with any other person so long as the marriage exists.

4. Its lifelong nature and exclusiveness are guaranteed by contract.

So, even at a natural level, divorce, adultery, and "homosexual marriage" are not compatible with marriage, and a lack of commitment means that no marriage has taken place.

A Supernatural Institution:   

In the Catholic Church, however, marriage is more than a natural institution; it was elevated by Christ Himself, in His participation in the wedding at Cana,(John 2:1-11), to be one of the seven sacraments

A marriage between two Christians, therefore, has a supernatural element as well as a natural one. While few Christians outside of the Catholic and Orthodox Churches regard marriage as a sacrament, the Catholic Church insists that marriage between any two baptized Christians, as long as it is entered into with the intention to contract a true marriage, is a sacrament.

In order to get married in the Catholic Church, you must be;

A Baptized Christian: Both partners do not have to be a Catholic in order to be sacramentally married in the Catholic Church, but both must be baptized Christians (and at least one must be a Catholic). Non-Christians cannot receive the sacraments. For a Catholic to marry a non-Catholic Christian, express permission is required from his or her bishop. A Catholic can marry an unbaptized person, but such marriages are natural marriages only; they are not sacramental marriages. The Church, therefore, discourages them and requires a Catholic who wishes to marry an unbaptized person to receive a special dispensation from his or her bishop. Still, if the dispensation is granted, a non-sacramental marriage is valid and can take place inside of a Catholic church.

Not Too Closely Related: Legal prohibitions on marriage between cousins (and other close blood relationships, such as uncle and niece) stem from the Church's ban on such marriages. Before 1983, marriages between second cousins were prohibited—indeed, former New York mayor Rudy Giuliani received an annulment of his first marriage after determining that his wife was his second cousin. Today, second-cousin marriages are allowed, and, under some circumstances, a dispensation can be obtained to allow a first-cousin marriage. The Church still discourages such marriages, however.