Pope John Paul II on modesty:

This does not, however, mean that physical shamelessness is to be simply and exclusively identified with complete or partial nakedness. There are circumstances in which nakedness is not immodest. If someone takes advantage of such an occasion to treat the person as an object of enjoyment, (even if his action is purely internal) it is only he who is guilty of shamelessness (immodesty of feeling), not the other. Nakedness as such is not to be equated with physical shamelessness. Immodesty is present only when nakedness plays a negative role with regard to the value of the person, when its aim is to arouse concupiscence, as a result of which the person is put in the position of an object for enjoyment. What happens then may be called depersonalization by sexualization. But this is not inevitable. Even when nakedness goes with mutual sexual enjoyment respect for the dignity of the person can be fully preserved.