This is a perennial question. It seems more cultural than biological, in the sense that many cultures are used to casual nudity. It is a peculiarly American perspective that tightly links social nudity to sexual interest and activity. The usual dynamic seems to be something like the following...
1. There is a combination of angst and excitement about the first trip to a nude beach or resort. Indeed, the angst is so great that the American club or resort policy is usually stated as "clothing-optional" and allowances are made for first-time visitors.
2. On arrival, there are all these people doing what everyone does on their front lawn, or by the pool, or at any other place--except they are naked. They have scars, wrinkles, sags, bellies etc. They look like everyone else does stepping out of the shower. The conversations are about weather, cars, sports teams, youtube cat videos...indistinguishable from what one hears at any July 4 cookout.
3. It occurs to the new people (if they are still dressed) that they are suddenly the exception. Clothes come off quickly, the etiquette of carrying and sitting on towels is rapidly learned and the focus becomes getting enough sunscreen on to avoid resembling a boiled lobster.
All of this is the antithesis of erotic. There is enough of the "will we fit in?" and "how do we introduce ourselves while naked?" angst and so on that sex is pretty far from anyone's brain. At most, people suddenly notice that it's a lot more comfortable being naked in the sun and in the pool or or on the beach.