Though I am a self-proclaimed Bangel, the truth of the matter is that Angel's subsequent relationships with Cordelia and Nina are counterintuitive to his entire rationale for abandoning Buffy in the first place. He left her because he did not feel worthy of her, he desperately wanted her to transcend his limitations.
So what is his justification for the Cordelia and Nina relationships? Don't get me wrong; I find Angel's affection for Cordelia to be touching and logical, given how close the two became. And Nina seemed nice enough. But what why did he attempt to start something with either of them?
Was he so desperate for companionship that he abandoned his nobility? Did he feel that they didn't deserve some normalcy?
It's fun to speculate.
Personally I have always felt that Angel made the decision that Buffy couldn't. It wasn't so much about nobility but doing what he had to. Throught the a large chuck of season 3 Buffy had tried, several times, to seperate herself from Angel. She tried to leave him, and found she couldn't. Joyce's words to him always stand out to me "I hope you can make the hard decision", he did and Buffy backed him, or at least understood why he was doing it. Buffy had learned the hard way that want and need are too very different thing.