Rumors at work (whether true or not) about your sexual activities could constitute sexual harassment. To have a viable claim, the harassment would need to be "severe and pervasive." A single rumor that fades away quickly probably isn't bad enough to create a hostile work environment, but persistent rumors over a longer period of time could alter the workplace and create a hostile work environment. You should keep written notes regarding anything that happens at work that may be relevant to this situation. Be specifice regarding date, time, location, what was said or done, by whom, and any witnesses. Don't jump to unfounded conclusions -- just stick to the observable facts. Also, you should inform your boss or human resources department about the rumors, and keep notes regarding your interactions with your boss and HR. Good luck.