Some fantasies are much better left as fantasies. They're not necessarily going to go the way you imagined and it could completely ruin things for you. For example, the couples recently written about on SexIs where the man 'accidentally' shot his wife or the man who strangled his partner. I don't have fantasies like that, but it seems to me that people weren't able to think as clearly when caught up in the heat of the moment and their fantasies turned into huge disasters. Then of course, you have things like threesomes where unexpected jealousy could ruin your relationship forever.