Why is seeing all of the breast more evil than seeing most of it?

Exactly which is more evil in the eyes of Americans, the breast in part sans nipple or the nipple? You see fashion and such were most pars of the breast is expose in some part or has tons of cleavage, seen greatly on the red carpet or the runway and even though is exposes a great deal of the breast it is seen as couture over trashy or slutty. It seems a woman can expose all sides of the breast and not be obscene but if any nipple is showing all hell breaks loose. Even though many may see cleavage or the wearing of a tube top etc. in poor taste but not obscene but if any nipple is expose they do? Why is the greater part of the breast being exposed tolerable but go to obscene when ever the nipple is exposed. To me it is as logical as saying you can kill people (which should not be done) so long as you do not do it in front of anyone.