I agree that Spike did not see it as rape in the moment and the lead up. It is in line from everything that happened before for Spike not to think of it as him forcing himself onto her. However, Buffy viewed the encounter as attempted rape from the onset. It is important to note that while Buffy has said no before she never fought against or struggled against Spike. While they had "rough sex" it was never shown to be preceeded with Buffy being held down against her will, screaming and trying to escape. All of their sexual encounters aside from the episode with Riley and this one start with Spike's initiation and Buffy's acception of it through inaction followed by her reiprocation. Moreover, the aftermath of the encounter soldified that what Spike did was try to force himself onto the person that he loves. That is why it was devesating to him, it validated that he is the monster everyone said he was.