Natural marriage
In Catholic canon law, natural marriage is the 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". It is distinguished from a sacramental or Christian marriage, in which the two parties involved are baptized.
Valid baptism a condition for Christian marriage
Since only the baptized can receive the other sacraments, the marriage of someone who has accepted Christian beliefs but has not been baptized is non-sacramental. Similarly, the marriage of a person whose baptism the Catholic Church judges to be invalid is a non-sacramental natural marriage. Examples of such baptisms considered invalid are those of the Mormons and the Jehovah's Witnesses.A marriage of two baptized Protestants, even if the church or churches they belong to and they themselves deny that marriage is a sacrament, and even if they contract marriage only civilly and not in church, is a sacramental marriage, not a merely natural marriage.
Transformation into sacramental marriage
The marriage that a non-baptized person, of whatever religion or belief, contracts, even with a baptized person, is a non-sacramental natural marriage. However, if the non-baptized person or persons are later baptized, the existing marriage automatically becomes sacramental and no longer merely natural.Conditions for natural marriage
If a Catholic marries a non-Catholic, the marriage is subject to Catholic canon law on impediments to marriage. If no Catholic is involved, the only impediments that apply are impediments affecting the very definition of marriage and impediments that are considered part of natural law.Any marriage that is non-monogamous, non-heterosexual, or involves non-humans is an invalid attempt at marriage according to natural law.