Is this film sexist?
I honestly think it's hard to say.
Obviously there's something twisted about chaining a half naked woman up. I know that Lazarus had the best intentions in mind. I understand that he was trying to cure her of her addiction, but at the same time I felt that if this were a man with the same problem this whole situation would not have happened due to a blatantly obvious sexual double standard in our society. That and most people probably don't want to see Samuel L. Jackson or even a woman chain up a half-naked young man for his sex addiction. Probably just wouldn't go over well unless it was a comedy or a porno or both. Boys WILL be boys, after-all.
On that note, the men who sleep with her are essentially taking advantage of her mental and emotional problems, but of course you don't see them being chained to radiator because you know... they're men. I also felt like the whole time she was chained up Ricci was being sexualized which is a little distasteful considering the subject matter. Another thing that bothered me was how she wore a chain as part of her wedding dress which basically implied that she was exchanging one form of bondage for another. One man to hold her down for another.
At the same time however this film most definitely humanizes a character that is otherwise seen as inhuman, i.e. the 'town whore'. Probably can't be regarded as feminist, but this is still a nice change from the norm nonetheless.