"The Christian idea of marriage is based on Christ's words that a man and wife are to be regarded as a single organism--for that is what the words 'one flesh' would be in modern English." Mere Christianity
However, the one-flesh idea of marriage wasn't just Jesus' idea. Jesus' words were based upon the statement in Genesis 2:24,
For this reason a man will leave his father and mother and be united to his wife and they will become one flesh.
According to the Judeo-Christian tradition, from the beginning of time God intended one man and one woman (not one man and one man, nor one woman and one woman) to come together as one flesh in lifelong marriage.
There are many things which seek to destroy the one-flesh nature of marriage in our modern society. And we must fight vigorously against all such enemies of what God has ordained. Of course, one of the most notable enemies of one-flesh marriage is the temptation to sexual intimacy and intercourse outside of marriage which is usually accompanied by the false promise of lifelong inloveness if one can simply find the "right" partner. Lewis addressed this false promise in the last essay he prepared for publication: We Have No 'Right to Happiness'.
In that essay Lewis makes several important points:
We have about as much "right to happiness" as we have a right to being six feet tall, a right to have a millionaire for a father, or a right to good weather when we want to have a picnic.
The right to "life, liberty and the pursuit of happiness" spoken of in the American Declaration of Independence refers to the right to pursue happiness by all lawful means. Whatever means of pursuing happiness which are lawful for some should be lawful for all.
We cannot build any society on the principle of freedom to pursue happiness by any and every means. This freedom must necessarily be limited if we are all to get along with one another.
What people today often mean by a "right to happiness" is a right to sexual happiness.
By promoting a so-called "right to sexual happiness" people today are putting the sexual impulse in a position of preposterous privilege. "The sexual motive is taken to condone all sorts of behaviour which, if it had any other end in view, would be condemned as merciless, treacherous and unjust."
When we are in the throes of sexual passion we are led to believe that we will go on being in love until death and that possession of the beloved will confer lifelong happiness. But, unfortunately, such is not always, or even often, if ever, the reality.
"When two people achieve lasting happiness, this is not solely because they are great lovers but because they are also--I must put it crudely--good people; controlled, loyal, fair-minded, mutually adaptable people."
If we are to maintain God's one-flesh ideal for marriage, we must then work hard at being those good, controlled, loyal, fair-minded, mutually adaptable people. For that is the only route to true and lasting happiness.
Father God, you know well enough that I am not that good, self-controlled, loving person you created me to be. I am often pulled, from moment to moment, by various passions, first this way and then that. Thank you for sending your Son into this world to live that perfect life I have failed to live. Thank you Lord Jesus, for dying on the cross for all of my fitful and failed pursuits of happiness. Thank you for rising again from the dead to breathe new life into my sexuality and into my marriage. Help me, by the power of your Holy Spirit, to live in one-flesh faithfulness to my spouse and to you. Help us to grow together in marriage as good, controlled, loyal, fair-minded and mutually adaptable people, to the glory of you, our Triune God. Amen.