In A Raisin in the Sun, why did Beneatha say she wouldn't marry George?

Beneatha experiments with different identities as all young people do.  At this point in the novel, she is interested in her African identity, something which she thinks George, as an "assimilationist," eshews (which he does). This is why Asagai has so much appeal to her for he represents that new (or original) identity in that he is an educated man from Nigeria, having a tribe to claim as part of who he is. The contrast between George and Asagai represents a theme in the novel concerning what direction the Younger family should go as they move toward the future.  Indeed, Walter Lee's chief dilemma is discovering who he is, finding an identity as a black man in a society that denigrates that identity.  Langston Hughes' poem, which the title of the play alludes to, engages this topic through the metaphor of a dream.