The reason plant matter takes longer to rot away is because there are no native fauna that feed off of it, we have no natural banana and orange feeders in the UK as we have no orange groves or banana trees for them to feed off. It will go eventually, sure, but remember a paper bag is simply wood pulped, so has some organic matter value.
As a slight diversion, the best way to deal with solid human waste is to carry it out and dispose of appropriately or..........smear it out across a larger area nice and thin! It's a bit Bobby Sands Dirty Protest, but it will all be gone in absolutely no time at all! Plenty of pooh eaters out there!