I opened up Lewis' Mere Christianity this afternoon to the chapter titled "Sexual Morality." I wondered how Lewis would approach the topic, considering that he published this book almost 60 years ago, in 1952.
Lewis begins with an exploration of modesty. I was surprised -- and pleased -- by his conclusions:
The Christian rule of chastity must not be confused with the social rule of 'modesty' (in one sense of that word); i.e. propriety, or decency. The social rule of propriety lays down how much of the human body should be displayed and what subjects can be referred to, and in what words, according to the customs of a given social circle. Thus, while the rule of chastity is the same for all Christians at all times, the rule of propriety changes. A girl in the Pacific islands wearing hardly any clothes and a Victorian lady completely covered in clothes might both be equally 'modest', proper, or decent, according to the standards of their own societies: and both, for all we could tell by their dress, might be equally chaste (or equally unchaste). Some of the language which chaste women used in Shakespeare's time would have been used in the nineteenth century only by a woman completely abandoned.
Lewis also points out that someone who breaks these social standards is not always being immodest:
When people break the rule of propriety current in their own time and place, if they do so in order to excite lust in themselves or others, then they are offending against chastity. But if they break it through ignorance or carelessness they are guilty only of bad manners. When, as often happens, they break it defiantly in order to shock or embarrass others, they are not necessarily being unchaste, but they are being uncharitable: for it is uncharitable to take pleasure in making other people uncomfortable.
Lewis is also very realistic about the differing standards in society. Having grown up in the conservative Christian culture, I've seen what a stumbling block the issue of modesty is for many people -- and I don't mean men who struggle with lust.
Men and women alike too often judge others based on the way they dress. Conservative women take pride in their own standard and look down on women who are "less modest." And on the other hand, women who come from more relaxed backgrounds sometimes judge their conservative counterparts as being legalistic.
I do not think that a very strict or fussy standard of propriety is any proof of chastity or any help to it, and I therefore regard the great relaxation and simplifying of the rule which has taken place in my own lifetime as a good thing. At its present stage, however, it has this inconvenience, that people of different ages and different types do not all acknowledge the same standard, and we hardly know where we are. While this confusion lasts I think that old, old-fashioned, people should be very careful not to assume that young or 'emancipated' people are corrupt whenever they are (by the old standard) improper; and, in return, that young people should not call their elders prudes or puritans because they do not easily adopt the new standard. A real desire to believe all the good you can of others and to make others as comfortable as you can will solve most of the problems. (emphasis mine)