There is no way of controlling 'these things'. In actual fact, I have always found it better to 'fight fire with fire' so to speak. If you try to deny your desires or fantasies, it makes them more potent. If you spend some time alone thinking about it (not in a cold shower). There's no need to be ashamed at all. Having fantasies about other people does not equate to cheating, and it doesn't matter whether those are men or women. : )