Lewis: Is Sexual Immorality the Worst Sin?

In September 1954 C. S. Lewis responded to a letter from a woman who had asked, 'Why has sex become man's chief stumbling block?' He wisely responded--

But has it? Or is it only the most recognisable of the stumbling blocks? I mean, we can mistake pride for a good conscience, and cruelty for zeal, and idleness for the peace of God, etc. But when lust is upon us, then, owing to the obvious physical symptoms, we can't pretend it is anything else.

Is it perhaps only the least disguisable of our dangers?

--Collected Letters of C. S. Lewis, 3:510; emphasis original