Book 3, Chapter 5: Sexual Morality
Lewis talks about the virtue of chastity. Christians believe that sex is good. After all, God invented sex. What’s bad is too much sex, or sexual obsession, or sexual perversion. There is something terribly wrong with us (with our nature) because of our perverted sexual desires. It’s very unnatural. The need for sex is natural, but the desire for too much of it is unnatural.
These are the things that lead to sexual immorality: (1) our fallen nature, (2) temptations by demons, and (3) advertisements/propaganda on TV.