Jesus was quoting the Word of God.

He was winding people back to the His original intention for marriage- as He says - not so from the beginning. 

Marriage is a foretaste of the union He wishes to share with us not just in the hereafter but today/now.

Reconciliation - the Sacrament - is union/communion.
Thanking someone is an act of union.
Saying sorry is an act of union.
Engaging in marital intimacy - bringing all our gifts into this act - is an act of union with God and His will, which is expressed in Word and in our bodies. 

Let's_set_aside_affectations;_affectations_distance_us_from_others_giving_in_to_a_vanity.