This particular quote from C.S. Lewis comes from Letter XVIII of The Screwtape Letters. Screwtape, a "senior undersecretary" devil, is writing to his nephew, Wormwood, on the subject of sex and sexual temptation. After talking about the concept of "love", and how it is "gratuitously associated" with sexual desire (and how hard that makes the devils' job), Screwtape continues:
Now comes the joke. The Enemy described a married couple as "one flesh". He did not say "a happily married couple" or "a couple who married because they were in love", but you can make the humans ignore that. You can also make them forget that the man they call Paul did not confine it to married couples. Mere copulation, for him, makes "one flesh". You can thus get the humans to accept as rhetorical eulogies of "being in love" what were in fact plain descriptions of the real significance of sexual intercourse. The truth is that wherever a man lies with a woman, there, whether they like it or not, a transcendental relation is set up between them which must be eternally enjoyed or eternally endured.
From the true statement that this transcendental relation was intended to produce, and, if obediently entered into, too often will produce, affection and the family, humans can be made to infer the false belief that the blend of affection, fear, and desire which they call "being in love" is the only thing that makes marriage either happy or holy.
"Mere copulation, for him, makes 'one flesh'" is doubtless, as others have pointed out, a reference to 1 Corinthians 6:16:
Do you not know that anyone who joins himself to a prostitute becomes one body with her? For "the two," it says, "will become one flesh."
Paul is not talking about marriage yet; he does that in chapter 7. Here he is simply talking about sexuality and sexual activity in general, yet he pulls in the very same text that is usually used to speak about marriage. This is why Screwtape says that "mere copulation" makes "one flesh".
As far as "eternally enjoyed or eternally endured": I don't think that Lewis (or Screwtape) intended to say, contrary to Luke 20:35, that "a man [who] lies with a woman" is married to her eternally (i.e. even in Heaven). That doesn't seem to be what he's talking about; he's talking about how the relation between the man and the woman which is caused by the act of sexual intercourse can be built into a happy, holy marriage, or can be used to break down a person's concept of love and marriage:
[If humans are led to misunderstand the real significance of sexual intercourse] humans who have not the gift of continence can be deterred from seeking marriage as a solution because they do not find themselves "in love", and, thanks to us, the idea of marrying with any other motive seems to them low and cynical. Yes, they think that. They regard the intention of loyalty to a partnership for mutual help, for the preservation of chastity, and for the transmission of life, as something lower than a storm of emotion. ... [Furthermore] any sexual infatuation whatever, so long as it intends marriage, will be regarded as "love", and "love" will be held to excuse a man from all the guilt, and to protect him from all the consequences, of marrying a heathen, a fool, or a wanton.
This, in Screwtape's and Lewis's eyes, is the mistake that will have to be "eternally endured"—because it can lead to a situation in which it's difficult or impossible to live as a Christian. Screwtape gets into this in more depth in letter XIX.