This is mostly cultural programming, though as you can see with your cover photo, cleavage mimics the curves of a woman's backside, which are also culturally seen as sexual. Younger people don't remember Playboy and Penthouse, where young men used to see their first sexual images. Such programming works its way into the general culture as these young men grow up to be ad executives and naturally learn to use the power of sex and sexual images to sell prod;)