Tznius/ modesty does not only apply to women but to men too. It is a shared obligation. Both men and women are expected to guard their eyes from exposure to violent or sexual images,and protect the purity of their minds and soul. Since men are more easily aroused and more likely to objectify women, men need extra guards to maintain that purity. Shmirat Einayim is one of the ways men do this. Women help men by dressing modestly in an external way, and they also protect their own internality by being 'attractive but not attracting!' Developing one's internality and covering the ego is the goal of tzniut for both men and women.