In a nutshell they're afraid of people using prompts generating black people looking like monkeys/gorillas. And other such sensitive examples that others have posted/mentioned.
It's not about prudishness, although generating pornography is one of the concerns. In the paper there's a full page on ethical concerns, but some of issues they mention are misinformation, and perpetuating harmful cultural stereotypes around race and/or gender roles.
Stable diffusion was un-neutered within 24 hours of its public release and the worst people do with it is Emma Watson porn.