I can't understand why people sexualize nudity.

Nudists often like to say that nudity is not sexual in nature. While I understand the intent behind this statement as a reaction to prevailing attitudes about nudity, which range from naive to ridiculous, I also don't fully agree. Nudity is not 0% sexual. And there's nothing wrong with that. There can be situations, very many situations, in which two naked people have zero sexual feelings towards each other, sure. But that also doesn't mean that people who spent a lot of time being naked, and being around other naked people, can't possibly feel arousal just by seeing naked flesh. It depends on the people involved, and the situation. If the right set of ass cheeks comes walking by, most people will take notice at the very least. And it does make a difference whether those buns are covered by a loose skirt, tight denims, or nothing.
But that shouldn't trouble us. There's a latent sexuality inherent to life. We don't have to guard ourselves against every little outcropping of sexuality. And we shouldn't guard children against each and every sexual feeling that might be awakened in them. That will only make them more vulnerable later. The things to be wary of are excess and exploitation.

We were born naked, and we were born without an instinct to wear clothes. We all know this.
Naturism/nudism has been around for long enough that we know that these people aren't on average more lascivious than the rest of us. Don't look at people online as examples, like the ones you find on this platform. Many of them probably aren't even really nudists, they're just fascinated by their own romanticized idea of nudism. It's just another fetish for them.