I didn't think I would be able to find it so fast, but I found it on this site: http://www.cslewisclassics.com/rgg.html
Here is how they quote it from MERE CHRISTIANITY:
Lewis ends the chapter "Sexual Morality" with a remarkable assertion: "
a cold self-righteous prig who goes regularly to church may be far nearer to hell than a prostitute" (p. 95). Why does Lewis consider spiritual sins to be worse than sins of the flesh? What is Lewis's view of the proper role of sexuality, pleasure, and chastity for Christians?
I can't remember in there what the exact quote is, but he does address this. He talks about how sin cannot exist in a vacuum... sin is always a corruption of what was once good by doing it in a manner it was not intended to be done. since all things originate with God, sin or Satan has no creative power. All he has is destructive (corruptive) power.
So it stands to reason that pleasure (of all forms) is a good thing, in the context for which God intended it. Outside of that it is a corruption, and sinful.