I've always thought that the vampire's personality starts out as being pretty much a carbon copy of the host's living self, but without the natural inhibitions the ensouled human would have had (Note Spike's dear, sweet little mother "coming on" to him almost immediately after being turned) If it feels good, they do it, if they want it, they take it. As for sex, incest rape, gender orientation, adult or child, none of that would or anything else mean anything whatsoever to the average vampire, and the linger they exist, the less and less of their host's personality traits remain.
As for Harmony and Spike retaining their mortal personalities, human Harmony and "Angel" Season 5 Harmony DO have personality differences, and comparison between milquetoast William and Spike by the time of "School Hard" in Season 2 "Buffy" results in the classic night and day cliche. (Almost certainly, Vamp William still had most of his human personality when he turned his mother, hence the revulsion at the suggestion of incest. Apparently his mother was an exception to the personality trait retention, unless she was more kinky as a human than William ever knew. Beyond that, probably most of Spike's initial bad ass act was designed more to impress Dru and Angelus than failure to retain his human personality traits)Give Harm another century or two, and I'm sure that we would see the same thing.