"And adult males have a huge bare patch of skin in the buttocks, testicles and perianal area"
I think the authors spared the monkey some embarrassment by only showing his face.
It has human-like facial features, which is creepy.
The title of your thread, initially, gave me the thought of an ugly faced mammal. However the second I saw the pictures two thoughts ran into my mind: these new species of monkeys are not ugly, and they have facial expressions that is synonymous with humans.
Now I see your point.