And it is, after all, safer and much better for your relationship to keep the fantasies in your head than acting on them. The problem with fantasies comes when they overtake one partner's desire for the other. And if a fantasy is absolutely necessary in order for one partner to become aroused or have an orgasm, then it's definitely a problem. The object of your fantasy might be someone you know in real life, which can be a problem if you are willing to act on the fantasy and cheat on your partner. Examine the way your fantasies play out - do they involve aspects that are missing from your relationship? If that's the case, then your relationship could be in trouble. Perhaps acting out your fantasies in a role-play fashion can spice up your relationship and ultimately save it.