Legality of nudity in National Parks
I found an article on Facebook at the link below. It is interesting to read, but prety much only ays that in most national parks the rules of nudity are pretty vague.
https://www.nationalparkstraveler.com/2013/04/view-overlook-nudity-and-national-parks23093
I think it woudl behoove the national parks system to adopt a policy similar to the State of Oregon, which in layman's terminology says "Nudity is permissible as long as you are not doing anything with the intent of sexually arousing either yourself or anyone else.."
Despite the State law there are many local ordinances against public nudity. However the Oregon Supreme court ruled that nudidty is a right, under the state constitution, and to the best of my knowledge anyone who has fought a ticket or arrest has had charges dropped based on that ruling. I'd suggest you research latest developments on your own before doing anything.
I suppose, the laws are still in place probably mostly to discourage those those sexual agendas AKA "swingers". At present they are just nuisance statures to help keep public order. Although nudity has been declared a right, public sexuality has not and is still very illegal.