In marriage a man and woman are so closely joined that they become “one flesh,” which involves spiritual as well as physical oneness. In marriage God brings a husband and wife together in a unique physical and spiritual bond that reaches to the very depths of their souls. As God designed it, marriage is to be the welding of two people together into one unit, the blending of two minds, two wills, two sets of emotions, two spirits. It is a bond the Lord intends to be indissoluble as long as both partners are alive. The Lord created sex and procreation to be the fullest expression of that oneness, and the intimacies of marriage are not to be shared with any other human being.
John MacArthur