I know this post is quite old, but I feel the discussion here is still very relevant, so I'll add my two cents worth.
Before I do so, however, let me just preface this by saying I am very much triggered by rape scenes in films, as many women in my family have been victims, and the long-lasting effects of those actions are much greater than anyone can imagine, even across generations. I don't like them as plot devices, and seeing them treated so casually by Hollywood writers leaves me feeling sick for days. I will also say that I am 100% a feminist, fight for equality constantly, and have furthermore had an attempted rape on me twice by the same individual. This is not a subject which I approach lightly.
All of this being said, I am still a Spike fan, and do give him a pass when I don't other characters in other films. Since this is both seemingly contradictory and extremely controversial let me explain.
First,there is the attempt itself. I think James Marsters is a fantastic actor, but he has made it known many times that he had a huge problem filming that scene and that it sent him into therapy, and I think it shows.
One of the reasons why Spike endured so long as a character was simply because the man portraying him is, I think, a much better person than the character was originally intended to be. Some actors wear masks, others pull out parts of themselves and we as the audience see it in the close-ups on the screen. The latter sort add depth to the characters they portray and make us love them. This was how Spike came to be a long running character, rather than a one-off villain.
Fast forward to the attempted rape. I can tell you my attempted attacker had no remorse in his face of any kind. I was inanimate to him, just a thing. The whole time I was being looked through, being disregarded. There was no connection of any kind. It was a senseless act. Perhaps attackers come off differently to different victims, but this is my personal experience with one, and the action was robotic. James, by contrast, looks in pain the whole time the attempt is going on. Spike, consequently, looks in turmoil, like the struggle is going on inside, even while trying to do something so horrible. He looks desperate, but not in a commanding in control sort of way, but in a pleading sort of way. The whole attempt looks like begging. My experience was that attackers don't beg; they tell you what to do.
Then, Buffy throws Spike, and as soon as the wall hits his head he's in full regret. It is almost as if the hit jogs some sense back into him, and he realizes what he is doing. He immediately regrets the last few minutes, and it shows on his face.
Later, in the crypt, Spike is disgusted with himself. He ponders why he tried to do something so terrible, then he asks why he couldn't. Vampires are evil, after all, and I think it is hinted throughout other episodes that pre-Buffy Spike had raped indiscriminately before. So, why the self-loathing and introspection now?
Many people cite Spike's definitive change happening after being re-souled, but I have felt that the attempted rape is actually where the change begins, and for all of the reasons stated above. Even while something so horrendous is going on, there are subtle changes happening to the character. That is both excellent writing and excellent acting, and I am often surprised that people cannot see it.
I'm not just deluding this perspective either, because the inspiration for this scene was in fact a real life incident with one of the writers, only it was a woman trying to force herself on her ex-boyfriend. As it has been described, the writer called it her worst day, the day that she talked about to no one, and the one she was most ashamed of. She felt if she and her boyfriend could just make love one more time they could get back together. She was desperate, and at some point lost sense of her boundaries and pushed too far. The boyfriend had to forcibly push her off and tell her to leave, just like Buffy did Spike. She felt low and horrible, and they used that incident for Spike, so we don't have to wonder about how Spike is feeling or thinking, because the real life model exists.
Does all of this still make it attempted rape? Yes. I will never argue that point. So, then why does Spike get to be excused?
The easy answer could be that Spike and Buffy are the work of fantasy, fiction, and are completely impossible in a real-world setting, so exceptions must be made for extraordinary circumstances. However, I'm not going to make this hole I've made for myself that easy to dig myself out of. Even in a real-world, non-vampire setting, I think I'd still forgive/excuse Spike, when I can't other characters or people. So, why?
The answer for me is in the execution. The writer who inspired the scene was pleading with her boyfriend, and Spike was pleading with Buffy. Pleading is not a controlling position, but rape is. At some point, one can cross over into the other. The pleading stops, and a sense of entitlement takes over. That sense of entitlement is where the rape happens, when your will becomes the only thing that is important. Spike starts to go there. The writer obvously started to go there, but they didn't stay there, and their realizations and self-horror are not long in coming, but immediate. How committed to the act of forcing your will on another can you be when the moment is fleeting? How many of us have had horrible fleeting thoughts? Ever been so mad you thought you could kill? Even for half a second? How much of a mass murderer are you at the end of the day?
It is a debatable point, but I feel that if Buffy had been unsuccessful in stopping Spike, the realization would have eventually come before the damage was done. Rapists don't care if their victims cry. They don't care that they hurt them. They are nothing to them. Spike clearly cares, which is huge if you're talking about someone who used to be a rapist. Buffy's tears mean something to him, and they do the impossible, make an evil creature realize he doesn't want to do evil anymore. It is a very different kind of attempted rape scene than any show I've ever seen, or experienced, vicariously or otherwise.
By contrast, let us look at another scene in the series, and one that, to me, seems more like rape than the actual rape scene, and that is when Spike tries to bite Willow just after being chipped. He barges in, struts about, claims her screams are a bonus, throws her to the bed, and completely disregards her. This is evil Spike. This is also what this looked like in real life, to me. The second attempt my attacker made on me, he was clearly enjoying himself, while I still remained just a thing.
Joss Whedon has stated many times his original premise for the show was to show real world problems in the shape of actual monsters. Spike may have been going for a bite on Willow, but to me, it was always a stand-in for a dorm room rape. This being the case, the actual attempted rape in "Seeing Red" had more soul in it than any of the previous episodes or seasons had. It was transformative, even before Spike went to see a demon about a girl.
Secondly, as for people saying Buffy is a victim during her tumultuous sexual relationship with Spike, I vehemently disagree. Yes, she is broken in season 6,and in a healthy real world scenario, should have eschewed a relationship with anyone until she got professional help, but to say she is a victim is, I feel, a bit far. Buffy is, at worst, confused about herself and her feelings for Spike and the world around her. I also see in her someone who wants to be excused for feeling the way she does about the world around her and the bad boy conveniently makes that happen. If he wants sex, well, then she can just excuse herself of all the culpability of wanting the same. It is worth noting that the bad boy trope, as well as a fantasy of rape, has been cited as a real thing women sometimes engage in because both limit their culpability for their own desires. I don't think Buffy asks to be raped, or in any way wants it in Seeing Red, but I do feel her sexual relationship throughout the rest of the season fits very much into that way of thinking.
Anyway, here is my explanation.