It depends . . . do you ALWAYS fantasize about someone else? Is it always the same person? Does that mean you might have feelings for another person? In that case, it's probably bad. I don't fantasize about other people when I'm with my guy. It doesn't work that way for me. When I'm watching 'ER', I'll sometimes wish Luka was my partner but Luka doesn't enter my mind when I'm making love to T.