Also, neither of these girls seems to be particularly promiscuous. Each of them only sleeps with one person in the entire story.
The word used in the title is 不純 (fujun), which can mean: impure, adulterated, foul, mixed, and dishonest. I think the "impure" meaning was more what the author was going for, basically "the opposite of innocent", aka highly sexual. I feel like that's probably what the translator meant to convey with "promiscuous", but they just misunderstood what the word really meant.