Jesus contrasted the Pharisee"s view of marriage with God"s view of it. God instituted marriage. It involves the union of a male and a female that results in a uniquely close relationship, a "one flesh" relationship. "One flesh" is a Semitic expression that means "one." [1] This relationship is closer than even the parent child relationship. Furthermore it continues throughout the rest of the husband and wife"s lives. [source][source][source]
"The import of all this is that marriage from its very nature and from the divine institution by which it is constituted is ideally indissoluble. It is not a contract of temporary convenience and not a union that may be dissolved at will." [2][source]
"While the spiritual element is vitally important in marriage, the emphasis here is that marriage is a physical union: the two become one flesh, not one spirit. Since marriage is a physical union, only a physical cause can break it-either death ( Romans 7:1-3) or fornication ( Matthew 5:32; Matthew 19:9)." [3][source]