CS Lewis--
We must now consider Christian morality as regards sex, what Christians call the virtue of chastity. 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. 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. 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, or 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.

also see:

http://www.sovereigngraceministries.org/pdf/teaching/modesty_heart_check.pdf