It seems obvious in some contexts . . . Anyway, the city council got some complaints that the bikini bottoms of the lifeguards were too revealing and perhaps would running afoul of the law to not show the butt!
The city council thought about things and the state law does not criminalize showing the butt, at least in the context of being at the beach or park.
Anyway, the city council of New Hanover took a surprising tact to the complaints of the female lifeguards showing too much skin. The council deleted from their local law the word butt or buttocks. There are some beach towns in North Carolina which have no anti-nudity ordinance and which simply allow for the application of the state law.
Some people were not happy but it seems that a lot of people considered the concern or controversy to be pointless. They were happy with the lifeguards saving lives in whatever they were wearing.
My guess is that sooner or later the city council of New Hanover might end up choosing to adopt some law similar to that of Washington state or Seattle, meaning, to not knowingly cause reasonable affront.
For what it matters, when I checked on some of the local laws re IE, such as those of Bellevue or MI, I think that the word buttocks is not included as being prohibited to show. Of course, there are lots of different contexts in which the butt may be shown or not and the context is either going make something more offensive or more ordinary.
It appears that men and women sunbathers could wear a thong while sunbathing on MI or in Bellevue, but it is hard to say for sure. In several years someone might find out.
In the meantime, it seems you can wear some sheer pants, but an open display of the butt would naturally, if done without some good cause such as art or recreation, lead to questions of causing reasonable affront.
Anyway, sometimes the city councils of various cities change the law to reflect changes in public norms and expectations. There probably is the Sports Illustrated sold for sale in some grocery and other stores and everybody probably can live with that . . . so . . . the city council went with newer public norms.