A man shall leave his father and mother and be joined to his wife, and the two shall become one flesh. So they are no longer two but one flesh.  Mk 10:7-87

The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament.
Catechism of the Catholic Church, 160107