Specific story: Dean gets assaulted and possibly raped by a spirit. I really don't remember much about it, but I believe there was a scarf involved in the teasing. It may have been pre-series, and Dean might have been dressed like a girl? I know it wasn't posted recently. Ring any bells?
General search: Any other stories where Dean is sexually man-handled by spirits or invisible assailants, such as
Filthy Mind.