Okay, Aeris isn't the pure definition of a slut, but she is slutty and, as I said, anyone who would act like that around someone they'd only just recently met would be considered to be infatuated at best. She never gives off any sign to me that she is after anything but a quick snog and shag. Possibly just trying to steal Tifa's guy, I dunno. But she certainly has no reason to be in love with him at all aside from that he might remind her of another guy - and that's not love for Cloud, that's love for Zack. So there you go.