God's giving and man's receiving is the basis for marriage. A man will leave his father and mother and be joined to his wife, and the two will become one flesh. That's more than simply a figure of speech. Jesus repeats it for emphasis. They are no longer two but one flesh.
Marriage is the closest communion that can possibly exist between two people.
It is the complete and committed union of a man and a woman into one - physically, emotionally, spiritually. Marriage is the only proper and healthy place for sexual love. Marriage is intended to be lifelong union. Only within a lifelong union can two people be completely open to each other without fear of one or the other leaving. It is completely unique.