sex rules, use your God-given tools
how sex works as a narrative tool, what does sex symbolize, how does sex drive the narrative forward/develop the characters, etc." at upupa_epops' The Free-For-All Meta Comment-A-Thon
Violence is separation, sex is connection, though these two become increasingly complex as the series go on. The series is built on fear primarily, and the flip side of fear is desire -- running from and running to. If the goal of BtVS the series is to find a way out of the Hellmouth (though it's not known to the characters at the beginning of the series), to conquer the very concept of fear and the *separation* that that implies (the separation between oneself and others, between oneself and the truth, between oneself and one's authentic self), then part of that means determining, eventually as close to fearlessly as possible, what one actually desires.
Authentic desire is as slippery and difficult as fear -- and desire for other people and for emotional connection with others manifests sexually in a lot of ways throughout the series, with (primarily) the friendship among the "core four" characters requiring the greatest purging of sexual desire and competition in order to get to a state of connection. Xander's sexual attraction to Buffy is about his desire to have a hero and a strong woman in his life; Willow's attraction to Xander is her desire to stay safe and secure in a world she knows to be difficult and painful. Ultimately the sexual component of these relationships has to fade in order to get past the surface reasons that the characters are attracted to each other for them to value each other fully as individual but separate beings.
The difficulty in forming a sexual relationship that does not lead to falling into the other person is emphasized throughout the series. Connection is necessary and one of the purposes of life, but sex is dangerous because it is a moment of pleasure and interdependency that is powerful and easy to lose oneself in. Just as violence is necessary within the show's story-world to defeat and eliminate demons representing one's fears, but it is possible to take this violence too far and lead to a destruction of potentially generative aspects of one's life and loves, sex represents a real possibility for the young (and some of the old, arrested-development vampire characters) to lose their identity, to different dramatic degrees. In connecting to another person and briefly at least believing the gap between them lost, they can lose their individual identity to the point where they are irrevocably altered. The transformation as the characters become older is to find, in sex, the loss of unhealthy boundaries which can only fall down when the boundaries between people fall down, which is something that is most easily done (but not solely done) during the intensity of sex, without losing the boundaries that help build a coherent identity.
As a result, sex is dealt with ambivalently in the show. Most dramatically, Angel loses his whole identity in sex, as a moment of perfect happiness is what is required for Angel's delicate construct of identity to fall into nothingness and for him to be tragically reborn having lost his sense of responsibility and propriety in losing for a few moments of bliss his constant feelings of guilt. This parallels the loss of "innocence" which Buffy undergoes in the episode -- which is and is not a loss of innocence -- the recognition not so much that she has wronged, but that a conception of herself that she may have had before the affair, that of her ability to know another person for a relatively short time and trust him completely in an intimate setting, is gone forever.
Relationships later in the show often involve characters losing their identities entirely in sex, from Willow and Tara becoming one in voice and deed in the famous spell-metaphor sex which is later commented on tragically in OMWF to Anya's defining herself primarily by the men in her life and punishing men often/primarily for infidelity to Riley deliberately seeking out (through metaphor) the loss of identity and separateness in vampires, because he doesn't feel that Buffy can ever have that loss of identity with him. This total loss of identity is shown as harmful, with Angel being the most extreme case but also Xander's pain at having been used by Faith (whose personal boundaries are so strong that she leaves herself completely unaffected emotionally by sex, as a defense mechanism, which is part of her constant wish to convince herself that she knows herself and her own desires completely, as a way of pretending that she has no fear), Willow's at Oz having a personal connection with Veruca she can never touch (and Oz' related fear about Willow's feelings about Xander). The clearest sexual dysfunctions are in season six, in which Willow and Tara's relationship (with some sexual components prominently noted) becomes so enveloping that Willow cannot conceive of an identity fully independent of Tara and so harms Tara to keep her with her, Buffy and Spike's mutually destructive relationship in which their whole identities become beaten and battered and the pursuit of sexual pleasure eclipses his genuine attempt to be a loving partner for her and harms her connection to her sister, friends and her duty.What is remarkable is that the series maintains an ambivalence about sex throughout its run, but one which ultimately tips toward the positive. Sex with Riley to some small degree and much more obviously Spike allows Buffy to go into the dark places in her soul -- where fear and desire, sex and violence intermingle, and there are parts of her which do not normally see the light of day which can only be accessed by dealing in a "safe" (albeit often actually unsafe) place with a lover. Buffy's Otherness from ordinary humans can only be explored through sex and violence, and it is in sex with Spike and the discovery of and eventual acceptance of parts of herself she had not wanted that she is able to find the will to live again -- which is what eventually allows her to regain her ability to connect to others non-sexually. Sex is a place where Willow finds that she can feel safe and express love with Oz; with Tara that she loves women and not men, which is a turning point which helps allow her to stop restricting her desires in an effort to conform with society; sex with Tara allows Willow to find a new self, and despite her initial belief that she cannot maintain this identity without Tara, her relationship with Kennedy and particularly sex with Kennedy, which does not end in world-ending disaster, is where she regains the ability to believe that she can maintain personal boundaries even if they can be more porous. The Xander/Anya love begins entirely with a mutual sexual attraction which was not meant to go anywhere and ends with Xander discovering an ability to view himself as a man and discovering the strength (c.f. The Replacement) within himself to decide to hold onto his life when he feels it slipping away; this love gives Anya, ultimately, the courage to stop blaming other people for her own sadness so much that she is willing to kill them. And Spike's relationship with Buffy is what allows the impact on his boundaries enough for him to, eventually, recognize that there is something of Buffy he needs for himself -- his initial attempt to build sexually an external soul in his joining with Buffy, and his moral failure when he tries to rape her in order to rekindle that connection, is what is the inspiration for Spike to regain his soul.
Ultimately, as with most of BtVS, sex is regarded in a manner that suggests the importance of moderation as well as understanding of what it entails. Sex is one of the places where the distance between people can be lowered much further than at almost any other times; where desire can be in principle expressed most fully outside the expectations of society, beyond the society of two. (The series generally is always about one-on-one couples; in principle larger societies could be possible.) There is always a risk of being injured when someone is unwilling to accept these desires, or the risk of losing track of some of one's own core beliefs and identity when losing one's boundaries and becoming one with another. However, the acceptance of boundaries which are stable but porous, and sex which can lead to becoming one with another without making it impossible to return to "oneself," somewhat changed but not totally and not instantaneously, is presented as a healthy and important part of the characters' attempts to become more authentic and more complete selves.