Classism.

Cultivators are supposed to be respectable and high status. Those women have bad karma, they do not live honorable and righteous lives, they cannot rise to the level of a cultivator and so they must only be able to drag them down. If you associate with morally unclean people you will become unclean too. It's really common for prejudiced ideas to be unthinkingly repeated from generation to generation without much examination on whether or not they have any basis in truth.

"Vell, I am not official," said Otto. "I do not haf zer sword und zer badge. I do not threaten. I am just a vorking stiff. And I make zem laff."

Vimes stared at the man. He'd never thought about that before. But yes...little fussy Otto, in his red-lined black opera cloak with pockets for all his gear, his shiny black dress shoes, his carefully cut widow's peak and, not least, his ridiculous accent that grew thicker or thinner depending on whom he was talking to, did not look like a threat. He looked funny, a joke, a music-hall vampire. It had never previously occurred to Vimes that, just possibly, the joke was on other people. Make them laugh, and they're not afraid.