I don't see what's objectionable about saying that Lily was an impetus for the Nina's transformation, both via her carefree spirit and her open sexuality; that seems like a pretty fundamental part of the film. The things Duncan mentions are manifestations of both of those character traits. One could attempt to argue against the transformation hinging
only on the effect Lily has on Nina, since the film provides plenty of other antagonisms, but I guess two-word contradictory posts are just as good as actually contributing to the discussion.
To be fair, I'm not sure I understand Duncan's objection either. For a young, sheltered person, drugs and sex seem like some of the most likely experiences to have profound impact of one's life - far more likely than seeing yourself sprouting black feathers. I'm surprised these are the things you'd object to, on what basis? Cliche?
I think one of the most interesting things about the film for me is that becoming the black swan seems like an insurmountable challenge since it's so incompatible with who Nina is. As Cassel points out to her in an early scene, some people have the ability to just let themselves go but Nina is too self-conscious and self-critical for that. However she is so driven that she actually succeeds by sheer force of will in changing herself - her character, her personality - in order to give the performance that's demanded of her. In my mind, the hallucinations are caused by this constant striving towards becoming a different person. When viewed in this light, it's not surprising at all that Nina goes mad and winds up destroying herself in the process; I think it's actually required since it's what she was actively working towards the entire film. I think personality is for the most part immutable, at least from conscious manipulation - how many "be a nicer person" New Years resolutions actually change the way someone thinks instead of just how they act (for awhile)? Nina has the technical proficiency, she acts correctly, but she has to affect a deeper, impossible sort of transformation to succeed.
I think this is a slightly different reading than seeing her transformation as something more positive: a self-actualization or an awakening of her sexuality to become a real woman, etc. If you take the film that way it's harder for me to see how to justify why Nina kills herself without it coming off as some negative comment on female sexuality, which I think is part of the reason some people have taken the film to be misogynistic.
One thing I only half-remember from the film - doesn't Cassel ask Portman if she's a virgin in one of the early scenes? I can't remember her response, or whether it seemed like that she was lying.