I thought the rape scene made sense from Spike's point of view, but, unlike a lot of others it seems, I thought the overreaction to it (both in universe and out) was ridiculous.
Here we have Spike, a soulless monster who spent over a century butchering his way through Europe. Spike has tried to kill Buffy and her friends numerous times, helped (at least to the Scoobies' knowledge) with Angelus' murder of Jenny, brutally tortured Angel to get a ring of ultimate power, and broke into Willow's room to kill/turn her (a scene made funny by Spike's "impotence", but was still about killing Willow.)
Okay, that's what Spike has done. All good, clean fun right? Well, then he tries to rape Buffy. That's his moral event horizon? Torturing and murdering people is all cool, but trying to rape someone who can easily defend herself isn't? Meh.
I agree that the over-reaction in the fandom is ridiculous, especially considering everything else that he and a bunch of other characters (Angel, Faith, Willow etc.) have done.
But I don't think it was in universe - because it wasn't treated as "moral event horizon". It was something that upset Spike enough to make him decide to get his soul back, but that was for another reason. When he comes back to his crypt, he is asking himself: "What have I done?...
Why didn't I do it?... What has she done to me?" and then goes on to conclude that he can't be either a monster or a man, which means he's nothing. In other words, he was shocked not just because he tried to rape Buffy, but because
he felt guilty about it. He was aware that, a few years earlier, he wouldn't have felt guilty in that situation; he might have just tried again. This was the resolution of his identity crisis going on throughout seasons 5 and 6 - he was neither here nor there, neither "monster" nor a "man", and was split between wanting to be badass evil vampire again and wanting to be good.
Now, just to future-proof myself, let me say that I found Edith's attempted rape scene in All in the Family quite powerful and disturbing. However, that's because Edith is a middle aged housewife and isn't a superhero who fights monsters every day of the week. In Buffy's case, an attempted rape really is just another Tuesday for her.
No, not when it's by her former lover, in a situation that starts in a disturbingly similar way to many of their their previous sexual encounters. She had feelings for him - which she finally admits in that scene - and despite saying she couldn't trust him
enough, she had placed quite an amount of trust in him (such as, not to kill her while she was sleeping with him and let him handcuff her) and was still willing to trust him on some matters even after that (to protect Dawn). I don't think she was really expecting that from him at that point. It's not the same as being attacked by someone random, it's a lot more traumatic and people can freeze in such a situation, despite their strength or fighting skills.
As for reactions of people in-universe, Xander and Dawn were acting angry and protective, but I wouldn't say that Buffy was treating it as "moral event horizon". Of course she was hurt and angry, but she tried to keep it to herself and not tell her friends, she told Xander to not go after Spike, she was still willing to entrust him with protecting Dawn and was inquiring where he had gone. She was concerned about him when she found him raving in the basement a few months later (in season 7 premiere). She only showed her anger when she was face to face with an apparently non-insane Spike, but it wasn't anything on the level of "die, you bastard" - she reluctantly talked to him and accepted his help, even though his accidental touch triggered a traumatic memory. I'm not sure what kind of reaction she could have had that you wouldn't see as an over-reaction, unless she was completely OK and unaffected by the AR.
The Scoobies were rather confused (and at times hypocritical) in their treatment of Spike - whether he should be treated as a soulless monster or a guy who could legitimately be a part of the gang, babysit Dawn, be invited to the wedding, etc. If he were nothing but a soulless monster, he should have been staked long time ago. And if he were to be treated as a man (and it's hard for the Scoobies not to see him in that light when he's in the capacity of Buffy's ex-lover), then it makes sense to condemn him for actions such as trying to rape his ex-girlfriend. It would be really strange for Xander and Dawn not to get upset at one of Buffy's boyfriends trying to rape her.
And in Dawn's case - she had been idolizing Spike in the teenage swoony way, not taking his past seriously, so this was a rude awakening for her, and it made it all the harder for her to be friendly with him again.
