Well I don't think it's that simple. I think the main issue is that seeing Riley again makes Buffy yearn for her old life, when she had that 'fire'.
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Except that she never really had that "fire" with Riley. Their relationship seemed to be much more "comfortable" than "firey." Faith in Buffy's body, offering Riley a more imaginative evening's recreation scared the boy so badly that he had to rush off to church the next morning, the first and only time on the series.
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I think it's not so much about Spike's duplicity although that didn't help. Also when confronted, he didn't say no I'm a good man etc. Firstly, he lied, then he said he "You know what I am, yet you come to me all the same.
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Spike said that he was keeping the eggs for a friend. The truth in that statement was never either proved or disproved. Maybe he WAS keeping the eggs for the Doctor, but we'll never know, because the subject never came up again. The "You know what I am..." statement sounded much more like someone saying, "You call ME stupid, but you hop into my bed at every opportunity, and you even come to me for comfort and help, so what, little Slayer, does that make YOU?"
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Say what you will about Spike or Spuffy but at this point Buffy couldn't be with him. It's too much of a moral chasm to go out on a nightly basis and kill soulless vampires and then sleep with one at the end of the night. It's also too much for to use someone (even if it is a demon) who has feelings for her. This is possibly the most important thing. It wasn't about comparing Spike and Riley but about using the opportunity to reassess the decisions she has been making of late. At this point even if she did love Spike she couldn't be with him, at least not in the way she has been. She can no longer hide in the dark with Spike, she needed to respect herself and demon who loves her. This is why she calls him by his human name. She doesn't want to hurt him but she has to be strong and break the cycle.
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Buffy told Spike that she COULDN'T love him, and it was killing her. That could just have easily meant that she COULDN'T love him, not because of what she felt, but because of the attitude of her friends. Remember that scene in "Gone," when Spike really wasn't doing pushups? Buffy was having a ball, no pun intended, because she could really feel alive again without having to worry about what her "friends" might say. If she had been visible when Xander walked in, it would have been, "I'm sorry...William," right then and there.
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I'm sure it makes sense to most, even me, but I just don't agree. I still thought Riley rocked. I can actually understand that it would be terrifying for him to see Buffy again and it couldn't have been easy after everything to see her with Spike but he was able to rise above that and tell Buffy what she needed to hear, also points for not being all that judgemental.