I like this post by Adam Serwer over at the Prospect on sex is Twilight (::shudders::), True Blood (which I'm working through now) and Buffy, up until the point where he says "Sex is 'bad' on [Buffy] only when the personal motivations involved are selfish--like when Buffy uses Spike for emotional and physical intimacy."
I think Buffy and Spike's relationship is more complicated than that. Obviously, Spike's attempted rape of Buffy is horrible, and it's clear that Whedon had a hard time dealing with the aftermath of it, which is one of the reasons I think the seventh season of the show had so much trouble. But in the relationship leading up to it, is the sex "bad"? I don't really think so. I've always found Buffy and Spike's relationship poignant (and, um, hot, though do I really have to tell anybody that?) because Buffy is incapable of acknowledging or truly absorbing that Spike really does love her. In "As You Were," when her ex-boyfriend Riley returns to Sunnydale married to someone else, the scene where Buffy seduces Spike, asking him to tell her that he wants her and he loves her, it's a terrible form of play-acting: Buffy wants to pretend its true, rather than acknowledging that it actually is true. The way Buffy treats Spike is awful: when she tells him "you're...convenient" after the first time they have sex, it's deliberately cruel. But I'm not sure that means that the fact that she and Spike have sex is a bad thing. Buffy's motivations are bad, Spike's much less so. They each get something they want out of it: they each hurt each other terribly. But ultimately, I think that's often what relationships are actually like, even if that's a very extreme example. And I don't think Whedon means for us to judge it.
One of the reasons I enjoy True Blood is that it has the most gorgeous credits sequence I think I've ever seen, alternating shots of dead things with scenes of Southern life in clubs and churches. And over all of it, Jace Everett snarls "I wanna do bad things to you." It's a threat, a promise, and an expression of intimacy. And I think that's basically what Buffy and Spike's relationship is about.