The following is a draft - think of it as an early beta. Bug reports welcome.
I haven't seen series seven (the last series) yet, nor have I seen Angel, so the following comments don't take these into account.Also, I've sen the fiulm, but I won't go into that. Lastly, some of the examples I use below count, I guess, as mild spoilers.
The prevailing morality changes over the six series, as the cast get older. In series one, they're all fifteen, and drinking, smoking, sex and drugs are all bad. Actually, I don't think there are any references anywhere in BtVS to illicit drugs, but I assume they're frowned upon (although there are obvious parallels with Willow's seduction by, overuse of, eventual dependency upon and virtual destruction by Wiccan magic).
Alcohol is here worth a mention, as the way its dealt with has parallels with the more complex issue of sex, and thus gives a convenient introduction to the genewral ethical framework of the series.
The only earlier episodes in which Buffy drinks alcohol (that I remember offhand) are:
1. in which the beer is cursed and turns her into a grunting cavewoman (wickedly sharp social commentary, that one, oh yes, if you're smart enough to get it) and
2. in which she goes to a frat house party, against the advice of her friends, and the drink turns out to be drugged, as part of an evil plan on the part of the frat boys to knock her out, sexualy molest her, and then sacrifice her to a big snake-thing in the basement (ahh! snake! Symbolism!).
After the characters reach twenty-one or so, suddenly beer loses its evil powers (removing the necessity for all those not-quite-convincing scenes where they got to nightclubs and drink coffee), and its okay for them to drink it in moderation, although still only Spike touches the hard stuff (Jack Daniels), just as he is the only smoking character, both of which (plus his fondness for black leather, industrial goth-rock and dodgy english accent) all serve as signifiers for his irredeemable evilness.
Basically, in Buffyland, no-one can have sex without something horrible happening. Especially if you're Xander, for some reason. Xander is engaged thoughout much of the series in an inner tussle with his manly, butch side and his uncool, jumper-wearing, friends-with-girls side. Whenever his manly side wins out he usually ends up being seduced and almost killed by a monster.
This may be because Josh Whedon, the creator and #1 scriptwriter of Buffy, is an overweight fantasy nerd who was probably an overwieght fantasy nerd during his high school years (evidence: his highly empathetic and knowledgable depiction of the Evil Trio - the three evil nerds in series six. Many of the in-jokes, star wars references, etc were evidence of a deep insider knowledge of geekdom). He probably didn't see as much action as he would have liked during his teenage years, and, in creating BtVS, has developed a self-justifying mythology where any sexual activity, or even attempts at sexual activity, or merely, in Xander's case, attempts at adopting a sexual persona (ie being a cool manly man rather than a jumper-wearing friend-of-girls) before the age of twenty-one or so is actually WRONG and EVIL and will be punished either by emotional upset or by scary monsters.
Below is a list of all the significant sexual relationships in Buffy that I can think of, and the various horrible things that happen to the participants as a direct or implied result of their sinful ways. Exclamation marks after names indicate that the people involved had actual naked sex rather than just going to the cinema together or whatever.
1. That nasty bloke in the first year of college! - seduces Buffy, turns to to be a dick in the end. Something bad happens to him, I forget what.
2. Angel! - cursed to turn super-evil every time he has sex with Buffy. In the end Buffy has to kill him. He returns from the dead, still evil, but she tames him by keeping him chained up with no shirt on and, most importantly, not having sex with him.
3. Riley! - goes okay for a while, but Riley feels emasculated by the fact that Buffy can kick his arse, and buggers off in the end.
4. Spike! - Spike and Buffy's relationship must be one of the most psychologically unhealthy and twisted ever depicted on popular television. Wayyy to complicated to get into here, but suffice it to say that they both spend most of the time wanting to kill each other, but every time they get into a fight they end up having sex, mostly standing up, mostly with their clothes on, as though to emphasise the basically unnatural nature of their relationship.
1. The only love interest I can remember is when she gets a ride with some older boys. They turn out to be warewolves and Buffy has to kill them all.
2. No others. number one seems to have scarred her too deeply.
1. Mantis creature posing as Jenny Callender, takes him home with promises of sexual delights, turns into giant praying mantis, tries to eat his head.
2. Inca Mummy Girl - Seems like a Latino hottie, turns out to be Inca Mummy Girl, tries to suck out his life force (in a bad, non-fun way).
3. Cordelia - bitchy stuck-up girl who doesn't like Xander and whom Xander doesn't like. Mostly palyed for laughs.
4. Anyanka! ex- vengeance demon. Longest-tem ralationship in the whole of Buffy, I think. Turbulent and troublesome, which is probably why Whedon lets it last so long - Xander doesn't need to be punished for his relationship with Anyanka, because the relationship IS the punishment. In the end, he stands her up at the altar, and they both feel terrible.
1. Oz! Turns into a warewolf, has an affair with anothe warewolf and has to leave to get his shit together or something.
2. Terra! Goes very well. Probably Whedon does not feel the need to punish them, as they do not have the necessary equipment to represent a threat to his masculinity. Anyway, at this point in the story, Josh is too busy punishing Willow for her over-use of magic (ie drugs).