A marriage between a baptized person and an unbaptized person is considered a natural marriage rather than a sacramental one.
It is called a natural marriage because by our very nature human beings have the ability to form the marriage bond with another person of the opposite sex.
Someone who is baptized, however, has an indelible mark on his/her soul. The soul is marked in such a way that every act of a baptized person, whether good or evil, is done precisely as a person marked as belonging to Christ. This means that when a baptized man and a baptized woman exchange marriage vows, they do so as belonging to Christ in a unique way, and even their very act of marriage becomes a Christian form of worship of God.
Natural marriage and sacramental marriage are both holy and subject to the same expectations. One key difference is that a sacramental marriage, because it is a living embodiment of Christ’s love for his church, is indissoluble whereas a natural marriage, for very serious reasons, can be dissolved. Sacramental marriage has an added theological dimension precisely because it is a commitment made by two persons marked for Christ.