They become one flesh...
Holy Scripture teaches us that man and woman were created for one another: "It is not good that the man should be alone..." The woman, "flesh of his flesh," i.e., his counterpart, his equal, his nearest in all things, is given to him by God as a "helpmate;" she thus represents God from whom comes our help. "Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh." The Lord himself shows that this signifies an unbreakable union of their two lives by recalling what the plan of the Creator had been "in the beginning:" "So they are no longer two, but one flesh
(Catechism of the Catholic Church 1605).