The natural argument is nonsense, but not because faeces are natural, more because the 'natural' construct is completely arbitrary. Most likely, everything you will ever come across is natural, there is no reason to create this weird idea that humanity is outside of nature, and I'm reasonably certain that the genesis of that idea resides in religion and man being part divine or in gods image and separate from the other beasts. People use this same nonsense reasoning when arguing against homosexuality.
Humans are a part of nature, everything we do is a part of nature.