The Catholic Church insists on modesty, for example. You won't find an outright ban on nudity, because what is considered modest can change depending on culture. However, if you are trying to remain chaste by avoiding pre-marital sex & masturbation, it is not difficult to see how sexually explicit images make that difficult. In fact, pornography is specifically singled out as a crime against chastity.

> 2523 There is a modesty of the feelings as well as of the body. It protests, for example, against the voyeuristic explorations of the human body in certain advertisements, or against the solicitations of certain media that go too far in the exhibition of intimate things. Modesty inspires a way of life which makes it possible to resist the allurements of fashion and the pressures of prevailing ideologies.

> 2524 The forms taken by modesty vary from one culture to another. Everywhere, however, modesty exists as an intuition of the spiritual dignity proper to man. It is born with the awakening consciousness of being a subject. Teaching modesty to children and adolescents means awakening in them respect for the human person. [1]

> 2354 Pornography consists in removing real or simulated sexual acts from the intimacy of the partners, in order to display them deliberately to third parties. It offends against chastity because it perverts the conjugal act, the intimate giving of spouses to each other. It does grave injury to the dignity of its participants (actors, vendors, the public), since each one becomes an object of base pleasure and illicit profit for others. It immerses all who are involved in the illusion of a fantasy world. It is a grave offense. Civil authorities should prevent the production and distribution of pornographic materials. [2]

[1]: http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2... [2]: http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2...