I have to concur with Ed. It seems to me that there is little point in creating art that generates no interaction between viewer and subject. This would apply equally to landscapes and nudes. In fact without interaction there is no art.
Nude studies are clearly erotic - otherwise they'd be called portraits. We are a society that clothes itself at all times and hence, disrobing is inherently erotic. If we deny that we deny human nature. If we were a society comprised only of nudists, (God forbid!) then in that context a nude would be merely a portrait, and a clothed person would be .......well hot! There is a clear difference between nude studies and pornography - although better, (much better) minds than mine, such as the US Supreme Court cannot articulate the dividing line here. Personally, I don't much care about the dividing lines my art appreciation is limited to that which elicits an emotional response on my part.