I think that what you're referring to is Riley's dalliance with the vampire ho and the scene with Faith and Wes in the vamp brothel, but correct me if I'm wrong. In both cases, the client was bitten on the arm or wrist, never the throat. The only other time that Buffy was bitten at that point was by the Master, and there wasn't anything sexual about it. Afterward, she was bitten, again in the throat, by Dracula, and, while his intention MIGHT have verged upon seduction, Buffy's reaction was only pain.
Traditionally, throughout fiction, and including Whedon's version of vampires feeding, the "sexual" bite is a far different thing from what the vampire does when actually feeding. Angel was feeding. Seduction never entered into it. Rape, possibly, but there was nothing gentle or loving about it. If Buffy couldn't recognize the difference, she is one sick little girl.
So I got very defensive at first, and wrote the post below, but then I re-read your post and realized you might not have meant Buffy was sick for being turned on, just that if she didn't get that it wasn't a loving moment, she was sick. So I wrote a angry, but not rude, post. I also think it had something to add to the discussion, so if anyone wants to read, it's below:
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That was even harsher. First off, Buffy was not sick, she was human. She didn't choose to get turned on and come. This is the same kind of loqic that scares boys and men who are molested to go to the police, because they think that since they're sometimes aroused by the molesters touches(depends on what kind of rape it is) it's consensual, or simply not something they can talk about. It seems to be the same kind of thing going on with Buffy; a special kind of feeding turns some, not all, people on(I doubt it matters if it's in the throat or arm or wrist). It depends on how their body reacts. It's not a choice, even though I imagine that some people can somewhat repress their bodies unwanting feelings, like some people can ignore pain.
I think I would also say, that Buffy's case is a bit different from Riley and the others (let's recall Giles said something about how many people found it pleasureable). Say that for most people, the pain takes over when bitten, specially if the vampire is brutal. Kind of like the difference between hitting and stroking someones flesh. But Buffy was a slayer, so her pain threeshold is higher. Her body can take a beating like ordinary humans can't. So a bite wouldn't physically hurt her as much. So the body mainly felt the pleasure. Then let's add the fact that she loved and trusted Angel, making her body even more responsive. If it had been Angelus, her feelings of helplessness and disgust with him might(again, this is also about how our bodies, not our minds, react) have somewhat lessened the pleasure. But it was Angel. Maybe she mainly reacted that way because she loved him and found him attractive, so her body was already used to think Angel=good feelings.
I do get what you're saying about it not intentionally being a sexual bite. It was a feeding bite. But that doesn't mean that Buffy's body had to react that way.
Now when calming down, I also see that maybe you didn't mean that she's sick because she was turned on, just that she was sick if she didn't see the difference between a "good" bite and a "bad" bite. In that case, I was a bit more defensive than I needed to be, since I might agree with you, depending on your exact meaning(I think Buffy got that Angel wasn't being sweet, just hungry). But I'll keep the post as it is, since I think I'm making a fair point. I'm going to add a note to the top though, to clarify.
Also, about examples of people liking to get feed on; Riley, the people Giles mentioned(he seemed to think it was common, unless I'm misstaken), the brothel in season 5 of Buffy and season 4 of Angel, Spike when he was turned (before the pain kicked in) and many occasions in season 8 and 9.
