In order to make my case, it must be established that extramarital sexual activity is wrong. Now since our sexual faculties are intrinsically oriented toward the generation of offspring as their final end, it can be reasonably inferred from this that any sexual activity which actively prevents procreation ought to be condemned as contrary to our nature and hence immoral. Since this is the case, it becomes a walk in the park to make the case for modesty. Since sexual activity is only licit within an appropriate context for the generation, bearing, and rearing of offspring (marriage), it would be against right reason to dress oneself in such a manner that would naturally bring about unwelcomed sexual desire or temptation in weakwilled men.
Therefore, modesty is not only an alternative lifestyle. It is not only a superior way of living that conforms better to the dignity of women than immodesty. It is a moral imperative, an obligation, a precept of the natural law that is written on all of our hearts. It flows naturally from our very ontology and should be something that everyone (male or female) should aspire for.