Why do naturists object to seeing the call of nature? It is one of the most natural things we do. Even the Pope and his celibate brethren urinate. Anyone out on a long hike will, sooner or later, feel the need to water the vegetation. I was on a big nude hike last summer which was being filmed for a documentary. When I went to relieve myself the camera crew asked me to warn them next time. They were young, female and as naked as the rest of us. Who could refuse? So I duly told them on the next occasion. They said "hold it while we set the camera up and check the exposure. I did, with difficulty. Then the camera rolled... . Who could object?
Short answer is that the natural human being most particularly with respect to waste production and excretion has been the subject of repeated historical social and religious (in particular) regulation as a subject of discourse and as an act both public and private, and deep rooted social stigmas exist. Being a naturist doesn't inherently change those attitudes, nor is the philosophy itself to the embodied self oriented, it's much more about the spirit and a sense of an almost moral or towards the good oriented coherence with nature and the self.