Girls don't like it when guys look at their ass as they're walking by. But then they rush out to buy shorts with a word plastered across their ass...what's up with that?

In a way, that's a powerful position to be in, right? But, more often, the incongruity you're noticing is just a result of our society's unwillingness to deal with sexuality in a reasonable way. So your example ends up being a microcosm of how sex operates in society, that is, through hypocrisy--the taboo accepted everywhere. Look at my ass, but don't look at my ass. If you look at my ass you're a perv, and if I show it off I'm a slut, so I'll show it off and you look, but we'll pretend I didn't and you didn't.