I didn't say it wasn't. Monos seem to be the men of this world. Men sexually assault and rape women with alarming frequency as it is. I would imagine those statistics are still similar in this world.How is this different from Natasha's behavior when she sexually harassed and almost raped Ashe?
Now imagine I told you there was a group of people even more sexually depraved than men. With vastly increased libidos and urges that they can't escape because it's a part of their DNA. I'm sure there's a lot more that goes into the why of rapists, but it isn't a strange idea that people might begin to fear this group.
Edit to add: No one in the game world is saying that only duplos commit sex crimes. All people within the game world are capable of that. They just believe that duplos are far more likely to commit those crimes.