I'm pretty sure that nudity itself is not illegal in most US states and can in fact be a protected right, counting somehow as free speech/expression I believe. Lewd conduct and other behavior considered antisocial is what technically violates the law. Though in practice this distinction seems to matter little unless you have a large group of nudists staging some kind of planned event. I witnessed a massive nude bike ride in Oregon recently that was fascinating. Thousands of people participated. While I don't really understand why people want to do it I have no problem with it and can certainly see the appeal in more appropriate locations, such as beaches. Nudity is entirely natural and the seemingly ever-present link between nudity and sex is purely cultural. Having said that, sex is also entirely natural and the seemingly ever-present stigma placed on sexual conduct is also purely cultural and unnecessary. I have no problem with seeing people naked and have no problem with watching people engaged in sexual activity. Nudity does seem impractical in an urban setting though, and if there is a legitimate sexual aspect to the nudity it is of course prudent to keep this activity away from children. I just try to base my opinions on pragmatism rather than abstract concepts of morality imposed on me.
“Today a young man on acid realized that all matter is merely energy condensed to a slow vibration, that we are all one consciousness experiencing itself subjectively, there is no such thing as death, life is only a dream, and we are the imagination of ourselves.” – Bill Hicks