In Buffy the vampire slayer vampiricism is a malicious force that can take over anyone, corrupt them, and cause them to turn on their friends. Xander and Willow's best friend Jessie has this happen to him. The question is what does this monstrousness map to in the “real” world. One likely example is that vampiricicism is a metaphor for older men preying on younger girls. The major characters are mostly teenagers, and all of the villains are ancient vampires that the hero can identify due to out of date fashions. They're trying to fit in with high school students, going to dance clubs with High School sophomores (the main cast) while they (the vampires) are much to old to be socializing with that age group. They then try to lure the young girls outside the club to metaphorically rape them. The leader of the vampires is the oldest and the most vile looking creature who is seeking a chance to go out into the world and take what is his.
Giles is not a problem because he accepts his proper place with the “scoobies”, his role as guardian and father figure. He also has romantic interests with age appropriate women (Jenny Carpenter and briefly Buffy's mother).
Jessie is an interesting case because he was one of the gang until he turned. This clearly symbolizes his growing sexual experience and how after he lost his innocence he was no longer one of the group.
Buffy seems to clearly take the stance that it is wrong for teenagers to have sex before senior year of high school. In later episodes Buffy having sex in her junior year turns out badly. Xander doesn't have a meaningful sexual relationship until his senior year. And Willow and Oz wait till their senior year to make sure they are both ready.