I think he was, at first, purely interested in her for her muse potential, but acted out on it in a way that would make anyone confuse it for romantic interest. Certainly he has a single minded focus on his work, and demonstrates throughout the movie that that is pretty much the only thing he cares about. The romance comes later, almost as an afterthought. It seems to me that the whole sleeping with her business was entirely optional, and contingent on some revealed quality of hers (I don't remember what precipitates the event).