I too have had this happen to me. For me, my father did in fact molest me so I think that is part of it. But I also think that a far higher number of humans have fantasies about sexual things that most would consider to be taboo, or even plain wrong. ITs when you come to a point where you feel an overwhelming urge to indulge this or any fantasy that you need to consider it to be an issue. Im not saying its not an issie and your being siilly - its obviously something thats concerning you and I can understand that as well. But i truly think you are ok here - there could be a million reasons for this type of fantasy, not all are problematic.