It's revealed she's actually a lesbian who was also in love with Ai (she could be bi? No indications otherwise yet). Her "promiscuity" is just a persona to those watching to make it seem they are living normal lives (Don't know about you, but college students ages 19-23 were very sex-driven and were happy to experiment. This seems normal.) In reality, they haven't had sex once and just used it as a cover, literally (bed sheets and steamed up windows), so they can have meetings on how to escape the city. They're friends, but unless a plot twist reveals something else, it doesn't seem to be headed in any other direction than that.