Jamieson, Fausset and Brown on Matthew 19:5

5. And said, For this cause--to follow out this divine appointment.

shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh?--Jesus here sends them back to the original constitution of man as one pair, a male and a female; to their marriage, as such, by divine appointment; and to the purpose of God, expressed by the sacred historian, that in all time one man and one woman should by marriage become one flesh--so to continue as long as both are in the flesh. This being God's constitution, let not man break it up by causeless divorces.