Why is female nudity both more and less acceptable in American society than male nudity?

For instance, most of the nudity in mainstream American movies is female. Yet, if a woman walks around without a shirt as a man might, it is illegal. If it is for entertainment in a film, then female nudity is acceptable, but if it is for function (if a woman is hot and takes off her shirt, or if she needs to feed and exposes her ), it is rude, distasteful, and possibly against the law. Anyone want to throw around some ideas why this is?