orchidcup says:

What constitutes indecent exposure depends on the standards of decency of the community where the exposure takes place.

These standards can vary from the very strict standards of modesty in places such as Afghanistan and Saudi Arabia, which require most of the body to be covered, to tribal societies such as the Pirahã or Mursi where full nakedness is the norm.

Even within a community, what will be seen as indecent will also depend on the context in which the exposure takes place. For example, it would be a reasonable expectation to see a naked person on a designated nude beach. However, even on a nude beach it may not be expected to witness explicit sexual activity.

The standards of decency have varied over time. During the Victorian era, for example, exposure of a woman’s legs was considered indecent in much of the Western world. As late as the 1930s, both women and men were expected to bathe or swim in public places wearing bathing suits that covered above the waist. An adult woman exposing her navel was also considered indecent in the West into the 1960s and 1970s. Today, however, it is quite common for women to go topless at public beaches throughout Europe and South America.

Although the phenomenon widely known as flashing, involving a woman exposing bare nipples by suddenly pulling up her shirt and bra, may be free from sexual motive or intent, it nonetheless is public exposure and is therefore defined by statute in many states of the United States as prohibited criminal behavior.

The motivation of the exposure is sometimes based on it being unusual, attention-getting, sexually arousing, or separately, as in a public policy protest, inappropriate and to show disrespect to the enemy side. The effects (including negative consequences) may be enhanced by intended or unintended publication of a photograph or film of the act, which would also include mooning.

Breastfeeding in public does not constitute indecent exposure under the laws of the United States, Canada, Australia, or Scotland. In the United States, the federal government and the majority of states have enacted laws specifically protecting nursing mothers from harassment by others. Legislation ranges from simply exempting breastfeeding from laws regarding indecent exposure, to outright full protection of the right to nurse.