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Natural marriage is the name given in Catholic canon law 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. ¯
 Natural marriage
 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 Muslim
Same as Catholic and Jewish.
 Catholic and Agnostic/Atheist
Same as above.
 Catholic and Buddhist/Hindu
Same as above
 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 sacramental.
 Sacramental marriages
 Between a Catholic and a Protestant
A marriage between a Catholic and a Protestant, including an Anglican (considered also a Protestant by the Catholic Church), is not only natural, but also sacramental, because both are baptized with the Trinitarian formula. If two Protestants marry, their marriage is already de jure considered to be a Catholic marriage, since both have received valid baptisms. However, non-Trinitarians such as Unitarians, Mormons or Jehovah's Witnesses are not united in sacramental vows.
 Between a Catholic and an Orthodox
Although there are some Trinitarian and other theological differences between Catholics and the Orthodox, a marriage between a Catholic and an Orthodox is considered sacramental because the theological differences are fairly minor. If an Orthodox marry another Orthodox, their marriage is already de jure considered to be a Catholic marriage, since both have received valid baptisms.
 Between two validly-baptized non-Catholics
The Catholic Church considers the valid marriage of two validly baptized non-Catholics to be sacramental by the very fact of their baptisms. The phrase "non-Catholic" here includes not only Orthodox and Protestants, but anyone who, in the Church's eyes, has been validly baptized.
 Gender rules
The Catholic church does not impose any matrilineal or patrilineal norms or requirements upon 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 (see who is a Jew? and conversion to Islam). These policy differences on what exactly constitutes a natural marriage have long shaped inter-community relations.
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.
 Invalid marriages
Any marriage that is non-monogamous (polygamy), non-heterosexual (same-sex marriage), or involves males under 16 years old, females under 14 years old (see marriageable age) or non-humans (zoophilia) is an invalid marriage in what the church calls natural law.