In 'Ways of Seeing' John Beger, the author, talks about how we see the world, how we perceive things and how we are taught to see things a certain way. There's a paragraph where he explains how or why women are seen as objects.

"One might simplify this by saying: men act and women appear. Men look at women. Women watch themselves being looked at. This determines not only most relations between men and women but also the relation of women to themselves. The surveyor of woman in herself is male: the surveyed female. Thus she turns herself into an object -- and most particularly an object of vision: a sight.”