It might be borderline if you are a strictly black coffee drinker, but just imaging the germs and mould that will take refuge in the milk traces of your flat white, latte or cappuccino?
Infectious disease expert Jeffrey Starke, a paediatrics professor at Baylor College of Medicine, tells The Wall Street Journal that even though there might technically be germs in an unwashed coffee cup, it's fine because they are yours. These germs, even if you're sick, won't hurt you again if you drink out of the same mug. "Very few viruses can exist for more than an hour on an inert object" Starke said.
Well, he is the expert.
I struggle to imagine doing it myself, but if i was in a shared office, i'd probably make alternative arrangements. Without being gross here, I have been witness to a shared sponge being used to mop up spills on the floor, then left at the back of the sink, to slowly and warmly grow whatever likes to grow on warm moist sponges. Any attempt at cleaning said slimy sponge is futule, because of the luke-warm hot water tap situation!
Pro Tip: make use of the microwave to boil the water and rinse your cup. Maybe boil that sponge too!