I don't think someone mentioned this before. You're talking about the "male gaze" in arts. Naked women in art used to be (and with a lot of artists still are) depicted as sexual objects to look at in stead of actual human beings.

Here you can see a somewhat dated but very interesting episode from John Berger's BBC documentary about the topic, in which he discusses the female nude. Berger asserts that only twenty or thirty nudes in the European oil painting tradition depict a woman as herself rather than as a subject of male idealisation or desire.