I believe that most people have at least one fantasy which they will always keep to themselves. Maybe it's a secret that's a bit taboo, or too twisted for most people to understand?
In my opinion, no one should ever feel obligated to share that side with anyone unless they're absolutely, 100% comfortable: spouse, lover, friend, whatever... You don't have to share.
I also believe the connection one has with their partner is of great importance. Not everyone shares the same level of intimacy, trust, communication, or time spent.
And even the most innocent and nonsexual fantasies: not everyone will understand. I know that my ex lovers were never understanding of my mind or my innermost thoughts. I held back a lot because they weren't on the same playing field.
Basically, if you're not confident that your partner will understand, then I would recommend holding off until you're sure he or she will accept it, especially if it's something so meaningful to you.
I was honest with my exes. I told them that I have a hidden, dark fantasy/secret. But I also let them know that they will never, ever, ever know what it was. It was my secret between myself and I. One which I would never speak of to another.
To anyone who says that is unhealthy, I challenge you. How is it unhealthy if you're aware of your demon and you keep it under control? Plus, my demon may not be equivalent to the next person's. It may very well be innocent. Regardless, I'm sane, I'm healthy, and it's nobody's business.