I'll begin with the obligatory disclaimer: I am not a lawyer, and absolutely no part of any of what follows is intended to constitute legal advice. I think common sense goes a long way with respect to getting naked where others might see you, but if you truly want to cover your ass (at least metaphorically), I would consult a lawyer. Generally, indecent exposure is usually classed as a misdemeanor, but there are often provisions for an escalation of penalties for repeat offenses (so called second or third strike laws), or for any offense committed in front of a child (variably defined). You never know who's going to come around that corner at the worst possible time; and while indecent exposure sometimes falls under the heading of "crimes against public morality and decency" (or something similar), I have seen a lot of cases where it's considered a sexual offense, even lumped in with obscenity, and cozying up obnoxiously close to prostitution laws (and worse). I don't think you need me to tell you what a drag it would be to end up on a sex offenders registry.
With that pleasantness out of the way (I'm a bit of an idealist on this point, but I lament that the image of people walking around naked that exists in most people's heads is one characterized by deviant perversion and not innocent purity, as it should be), I'd like to analyze some of the similarities and differences I've encountered in the indecent exposure laws (or closest facsimile) for each state, which I've transcribed below. While I cannot make any guarantee, I have tried to ensure that each code excerpt is accurate and up to date. For the sake of brevity, I've cut out all but the most salient points with regards to exposing one's genitals in public and/or in front of other people. I've included links should you want to read the full text. Most of these laws include additional provisions governing public displays of sexual activity (I figure that kinda goes without saying). Also excluded below are exceptions carved out for breastfeeding mothers, which I've encountered in the following states: Arizona, Arkansas, Florida, Idaho, Illinois, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New York, North Carolina, Rhode Island, South Carolina, Tennessee, Utah, Virginia, Washington, Wisconsin, Wyoming.
Now, on to the meat of the subject. There are many discrepancies across states about what constitutes "indecent exposure", in terms of which parts of the body we're talking about. Generally speaking, the genitals (or "sex organs") and anus (I feel like you really have to be trying, to expose your anus) are mentioned, but whether or not that includes buttocks and female breasts (thong enthusiasts and topfreedom advocates take heed) is not always clear. There is almost always a requirement that the offender's behavior either be conducted with sexual intent, and/or be performed under circumstances in which the offender does or should (sometimes it's one and not the other) know that his behavior will cause affront or alarm (or various other words for distress) to an unsuspecting and/or nonconsenting party. Which is why - as Florida (organized nudism capital of the United States) explicitly articulates - nudist gatherings, sufficiently insulated from public view, are usually permitted under law.
In some cases, there are separate provisions for private behaviors, criminalizing indecent exposure behind closed doors (or in front of open windows). Sometimes, but not always, there are exceptions made for spouses or, less frequently, co-habitators (providing a little leeway for nudism at home). As always, you should consult the full text of the law in your state before engaging in any potentially risky behaviors, and cross-reference it with local laws for the area in question - which may include even more restrictive provisions than the state code, as is the case in some parts of the reputationally free-wheeling state of Vermont.
All of this leads me to confirm my belief that, other than the "safety in numbers" approach of organized bike rides and protests, the safest way to get naked where others might see you is to stick to less populated areas (like remote hiking trails) - theoretically, there's no crime if nobody sees you - and to make an effort to cover up if somebody does happen upon you unexpectedly - demonstrating a lack of intent to cause affront or alarm. I know, it's tempting to want to spread awareness and try to normalize the occurrence of nude recreation. I get it. But I guarantee you this is not the most effective way of doing that. Consider that people can be made aware of your choice to engage in nude recreation without exposing yourself in a way that makes you criminally liable, or that callously disregards any consideration to the diversity of perspectives and sensitivities that exists among the population. In other words, you can make your point without swinging your dong in somebody's face, to say nothing of shoving it down their throat. And, on that subject, it goes without saying that the instant sex gets added to the mix, you're taking your liberty into your own hands (among other things ;-p).
Without further ado, here are the indecent exposure laws in all 50 states of the union:
Alabama: "A person commits the crime of indecent exposure if, with intent to arouse or gratify sexual desire of himself or herself...he or she exposes his or her genitals under circumstances in which he or she knows the conduct is likely to cause affront or alarm."
Alaska: "An offender commits the crime of indecent exposure...if the offender knowingly exposes the offender's genitals in the presence of another person with reckless disregard for the offensive, insulting, or frightening effect the act may have."
Arizona: "A person commits indecent exposure if he or she exposes his or her genitals or anus or she exposes the areola or nipple of her breast or breasts and another person is present, and the defendant is reckless about whether the other person, as a reasonable person, would be offended or alarmed by the act."
Arkansas: "A person commits indecent exposure if, with the purpose to arouse or gratify a sexual desire of himself or herself or of any other person, the person exposes his or her sex organs in a public place or in public view; or under circumstances in which the person knows the conduct is likely to cause affront or alarm."
California: "Every person who willfully and lewdly...exposes his person, or the private parts thereof, in any public place, or in any place where there are present other persons to be offended or annoyed thereby...such as is offensive to decency, or is adapted to excite to vicious or lewd thoughts or acts, is guilty..."
Colorado: "Any person who performs...a knowing exposure of the person's genitals to the view of a person under circumstances in which such conduct is likely to cause affront or alarm to the other person...in a public place or where the conduct may reasonably be expected to be viewed by members of the public commits public indecency."
Connecticut: "A person is guilty of public indecency when he performs...a lewd exposure of the body with intent to arouse or to satisfy the sexual desire of the person. ...For the purposes of this section, 'public place' means any place where the conduct may reasonably be expected to be viewed by others."
Delaware: "A person is guilty of lewdness when the person does any lewd act in any public place or any lewd act which the person knows is likely to be observed by others who would be affronted or alarmed."
Florida: "A person commits unlawful exposure of sexual organs by: exposing or exhibiting his or her sexual organs in public or on the private premises of another, or so near thereto as to be seen from such private premises, in a vulgar or indecent manner; or being naked in public in a vulgar or indecent manner. ...The exposure of sexual organs by...an individual who is merely naked at any place provided or set apart for that purpose...does not violate this section."
Georgia: "A person commits the offense of public indecency when he or she performs any of the following acts in a public place: ...a lewd exposure of the sexual organs; a lewd appearance in a state of partial or complete nudity..."
Hawaii: "A person commits the offense of indecent exposure if, the person intentionally exposes the person's genitals to a person to whom the person is not married under circumstances in which the actor's conduct is likely to cause affront."
Idaho: "A person is guilty of a misdemeanor if the person willfully and lewdly...exposes his or her genitals; exposes developed female breasts, including the areola and nipple...in any public place or in any place where there is present another person or persons who are offended thereby."
Illinois: "Any person of the age of 17 years and upwards who performs...a lewd exposure of the body done with intent to arouse or to satisfy the sexual desire of the person...in a public place commits a public indecency."
Indiana: "A person who knowingly or intentionally appears in a public place in a state of nudity commits public nudity. ...As used in this section, 'nudity' means the showing of the human male or female genitals, pubic area, or buttocks with less than a fully opaque covering, the showing of the female breast with less than a fully opaque covering of any part of the nipple, or the showing of covered male genitals in a discernibly turgid state."
Iowa: "A person who exposes the person's genitals or pubic area to another not the person's spouse...commits a serious misdemeanor if all of the following apply...the person does so to arouse or satisfy the sexual desires of either party...the person knows or reasonably should know that the act is offensive to the viewer."
Kansas: "Lewd and lascivious behavior is...publicly exposing a sex organ or exposing a sex organ in the presence of a person who is not the spouse of the offender and who has not consented thereto, with intent to arouse or gratify the sexual desires of the offender or another."
Kentucky: "A person is guilty of indecent exposure...when he intentionally exposes his genitals under circumstances in which he knows or should know that his conduct is likely to cause affront or alarm..."
Louisiana: "The crime of obscenity is the intentional...exposure of the genitals, pubic hair, anus, vulva, or female breast nipples in any public place or place open to the public view...with the intent of arousing sexual desire or which appeals to prurient interest or is patently offensive."
Maine: "A person is guilty of indecent conduct if...in a public place...the actor knowingly exposes the actor's genitals under circumstances that in fact are likely to cause affront or alarm."
"A person is guilty of indecent conduct if...in a private place, the actor exposes the actor's genitals with the intent that the actor be seen from a public place or from another private place."
"A person is guilty of indecent conduct if...in a private place, the actor exposes the actor's genitals with the intent that the actor be seen by another person in that private place under circumstances that the actor knows are likely to cause affront or alarm."
Maryland: "A person may not commit the common law crime of indecent exposure."
Massachusetts: "...persons guilty of indecent exposure shall be punished..."
Michigan: "A person shall not knowingly make any open or indecent exposure of his or her person or of the person of another."
Minnesota: "A person who...willfully and lewdly exposes the person's body, or the private parts thereof...in any public place, or in any place where others are present, is guilty of a misdemeanor."
Mississippi: "A person who willfully and lewdly exposes his person, or private parts thereof, in any public place, or in any place where others are present, or procures another to so expose himself, is guilty of a misdemeanor."
Missouri: "A person commits the offense of sexual misconduct in the first degree if such person...exposes his or her genitals under circumstances in which he or she knows that his or her conduct is likely to cause affront or alarm."
Montana: "A person commits the offense of indecent exposure if the person knowingly or purposely exposes the person's genitals or intimate parts...under circumstances in which the person knows the conduct is likely to cause affront or alarm in order to abuse, humiliate, harass, violate the dignity of, or degrade another; or arouse or gratify the person's own sexual response or desire or the sexual response or desire of any person."
Nebraska: "A person, eighteen years of age or over, commits public indecency if such person performs or procures, or assists any other person to perform, in a public place and where the conduct may reasonably be expected to be viewed by members of the public...an exposure of the genitals of the body done with intent to affront or alarm any person."
Nevada: "A person who makes any open and indecent or obscene exposure of his or her person, or of the person of another, is guilty."
New Hampshire: "A person is guilty...if...such person...exposes his or her genitals...under circumstances which he or she should know will likely cause affront or alarm."
New Jersey: "A person commits a disorderly persons offense if he does any flagrantly lewd and offensive act which he knows or reasonably expects is likely to be observed by other nonconsenting persons who would be affronted or alarmed. ...As used in this section...'lewd acts' shall include the exposing of the genitals for the purpose of arousing or gratifying the sexual desire of the actor or of any other person."
New Mexico: "Indecent exposure consists of a person knowingly and intentionally exposing his primary genital area to public view. As used in this section, 'primary genital area' means the mons pubis, penis, testicles, mons veneris, vulva or vagina."
New York: "A person is guilty of exposure if he appears in a public place in such a manner that the private or intimate parts of his body are unclothed or exposed. For purposes of this section, the private or intimate parts of a female person shall include that portion of the breast which is below the top of the areola. This section shall not apply to...any person entertaining or performing in a play, exhibition, show or entertainment."
North Carolina: "Any person who shall willfully expose the private parts of his or her person in any public place and in the presence of any other person or persons, except for those places designated for a public purpose where the same sex exposure is incidental to a permitted activity...shall be guilty."
North Dakota: "An individual, with intent to arouse, appeal to, or gratify that individual's lust, passions, or sexual desires, is guilty...if that individual...exposes the individual's penis, vulva, or anus in a public place."
Ohio: "No person shall recklessly...expose the person's private parts...under circumstances in which the person's conduct is likely to be viewed by and affront others who are in the person's physical proximity and who are not members of the person's household. ...Whoever violates this section is guilty of public indecency."
Oklahoma: "Every person who willfully and knowingly...lewdly exposes his or her person or genitals in any public place, or in any place where there are present other persons to be offended or annoyed thereby...shall be guilty, upon conviction."
Oregon: "A person commits the crime of public indecency if while in, or in view of, a public place the person performs...an act of exposing the genitals of the person with the intent of arousing the sexual desire of the person or another person."
"A person commits the crime of private indecency if the person exposes the genitals of the person with the intent of arousing the sexual desire of the person or another person and...the person is in a place where another person has a reasonable expectation of privacy; the person is in view of the other person; the exposure reasonably would be expected to alarm or annoy the other person; and the person knows that the other person did not consent to the exposure."
Pennsylvania: "A person commits indecent exposure if that person exposes his or her genitals in any public place or in any place where there are present other persons under circumstances in which he or she knows or should know that this conduct is likely to offend, affront or alarm."
Rhode Island: "A person commits indecent exposure/disorderly conduct when for the purpose of sexual arousal, gratification or stimulation, such person intentionally, knowingly, or recklessly...exposes his or her genitals to the view of another under circumstances in which his or her conduct is likely to cause affront, distress, or alarm to that person. Any person may be a complainant for the purposes of instituting action for any violation of this section. This act shall not apply to any conduct between consenting adults where the complainant is an unintended witness."
South Carolina: "It is unlawful for a person to wilfully, maliciously, and indecently expose his person in a public place, on property of others, or to the view of any person on a street or highway."
South Dakota: "A person commits the crime of public indecency if the person, under circumstances in which that person knows that his or her conduct is likely to annoy, offend, or alarm some other person, exposes his or her anus or genitals in a public place where another may be present who will be annoyed, offended, or alarmed by the person's act."
Tennessee: "A person commits the offense of indecent exposure who...in a public place or on the private premises of another, or so near thereto as to be seen from the private premises...intentionally...exposes the person's genitals or buttocks to another...and reasonably expects that the acts will be viewed by another and the acts will offend an ordinary viewer; or are for the purpose of sexual arousal and gratification of the defendant."
Texas: "A person commits an offense if he exposes his anus or any part of his genitals with intent to arouse or gratify the sexual desire of any person, and he is reckless about whether another is present who will be offended or alarmed by his act."
Utah: "An actor commits lewdness if...the actor performs...an act exposing the actor's...genitals; female breast below the top of the areola if the actor is female; buttocks, anus, or pubic area...in a public place; or under circumstances which the actor should know will likely cause affront or alarm to, on, or in the presence of another individual."
Vermont: "A person guilty of open and gross lewdness and lascivious behavior shall be imprisoned...or fined..or both."
Virginia: "Every person who intentionally makes an obscene display or exposure of his person, or the private parts thereof, in any public place, or in any place where others are present, or procures another to so expose himself, shall be guilty."
Washington: "A person is guilty of indecent exposure if he or she intentionally makes any open and obscene exposure of his or her person or the person of another knowing that such conduct is likely to cause reasonable affront or alarm."
West Virginia: "A person is guilty of indecent exposure when such person intentionally exposes his or her sex organs or anus...and does so under circumstances in which the person knows that the conduct is likely to cause affront or alarm."
Wisconsin: "Whoever...publicly and indecently exposes genitals or pubic area...is guilty. ...'Publicly' is susceptible to a construction that will avoid the question of constitutional overbreadth, by limiting the application of the statute to constitutionally permissible goals of protecting children from exposure to obscenity and preventing assaults on the sensibilities of unwilling adults in public."
Wyoming: "A person is guilty of public indecency if, while in a public place where he may reasonably be expected to be viewed by others, he...exposes his intimate parts, with the intent of arousing the sexual desire of himself or another person. ...'Intimate parts' means the external genitalia, perineum, anus or pubes of any person or the breast of a female person."
