Jesus goes back to the first marriage to see what was originally intended by God, at the beginning of creation verse 6. Marriage was given in this way before sin came into the world. God instituted marriage as a great blessing, uniting the male and female in his creation. It was given by God as a vehicle for giving and receiving love, for security, a safe environment in which children should grow up, and the only place for sexual activity. So they are no longer two, but one.
Marriage involves a public, exclusive commitment of a man and woman to one another.
It is public in that the man is seen to leave his parental home to set up one with his new wife. There is a change of priorities, and the relationship with the parents has to come second to the marriage relationship. There is a unity with the husband and wife becoming one flesh. There is an allusion to sexual intercourse here, but there is also the idea that the lives of the couple become so involved with one another that they are one. This also affirms the ideal of the permanence of marriage.