I think your comment about unnecessary debate is on target. Much of nature photography is pretty obvious, but the edges and areas of debate probably detract from the main idea which is to create great photos. Invariably the definition would come down to the size of the manmade structure in the image, or the size of the person.
There is some specific information on ethics and ethical captioning. It's probably more important to represent the facts behind an image. I've seen some fabulous images in PPA competitions that are purely digital creations - and correctly represented as such.
We have enough debate on whether captive animals or captive animal photos are okay.