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. The only thing closer is the relationship between Christ and the Church, between Christ and the believer. We are privileged in this congregation to have a few couples who have been united as one flesh for fifty years or more.