Natural marriage is the name given in Catholic Church canon law

Canon law

Canon law is the body of laws & regulations made or adopted by ecclesiastical authority, for the government of the Christian organization and its members. It is the internal ecclesiastical law governing the Catholic Church , the Eastern and Oriental Orthodox churches, and the Anglican Communion of...

 to the lawful union of a man and a woman from any type of religious background. Being defined in canon 1055, it is a legal pre-requisite to sacramental marriage or Catholic marriage

Catholic marriage

Catholic marriage, also called matrimony, is a "covenant by which a man and a woman establish between themselves a partnership of the whole of life and which is ordered by its nature to the good of the spouses and the procreation and education of offspring...

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Catholic and Jewish

A marriage between a Catholic and a Jew is natural because the Jewish partner is not baptized. If the partner is baptized, then the marriage is sacramental. Likewise, two married Jews have contracted a natural marriage.

Catholic and Mormon/Jehovah's Witness

A marriage between a Catholic and a Mormon or a Jehovah's Witness, even though the non-Catholic partner considers his or herself Christian, is merely a natural marriage because the second partner is not baptized in the name of the Holy Trinity. The Catholic Church requires a valid trinitarian baptism in order for the marriage to be considered sacred.

Catholic and Protestant

A marriage between a Catholic and a Protestant (be it Reformed, Lutheran, Methodist, etc) or Anglican (regarded as a form of Protestantism by Catholic doctrine) is more than natural; it is sacramental because both are baptized in the name of the Holy Trinity. If two Protestants marry, their marriage is already de jure considered to be a Catholic marriage, since both have received valid baptisms. However, non-Trinitarian Protestants such as Mormons or Jehovah's Witnesses are not united in sacramental vows.

Catholic and Orthodox

Although there are a few differences in Trinitarian theology for Catholics and Orthodox, a marriage between a Catholic and an Orthodox is considered sacramental because the theological differences are fairly minor. If two Orthodox marry, their marriage is already de jure considered to be a Catholic marriage, since both have received valid baptisms.

Gender rules

The Church does not add any matrilineal or patrilineal norms on the question of natural marriages. On the contrary, certain religions, such as Judaism and Islam, add requirements on the part of the female or male partner (cf Jewish mother and conversion to Islam). These policy differences on what exactly constitutes a natural marriage have long shaped inter-community relations.

Re-marriages

Re-marriages are specially allowed when a natural marriage becomes a sacramental marriage. However, if a Protestant converts to Catholicism after his second or third marriage, then the first marriage has priority since it is already sacramental and indissoluble. Under certain situations, re-marriage can be juridically similar to polygamy

Polygamy

Polygamy is a marriage which includes more than two partners...

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Un-natural marriages

Any marriage that is non-monogamous (polygamy

Polygamy

Polygamy is a marriage which includes more than two partners...

), non-heterosexual (gay marriage) and non-human (bestiality) is an un-natural marriage in what the Church calls natural law

Natural law

Natural law, or the law of nature , is any system of law which is purportedly determined by nature, and thus universal. Classically, natural law refers to the use of reason to analyze human nature and deduce binding rules of moral behavior. Natural law is contrasted with the positive law Natural...

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