People fantasize about EVERYTHING - from being rich to being with someone they find attractive. I think it's just the way the mind works. If in a relationship one person takes fantasy to the next level, then its wrong. Other than that, no harm done. Sometimes, I think people need to keep fantasies just that - a FANTASY - and keep it to themselves. Asking someone not to fantasize about other people or things is like asking someone not to imagine!
I hope you don't view this as too much of a cop-out answer, but the truth is that it's up to each couple to define for themselves what fidelity means. Open, honest, and non-judgemental dialogue will help you to establish with your partner how far is "too far." Again, sorry if this seems like a lame non-answer, but there's no one blanket statement that can be made to cover all relationships with their infinite subtleties and nuances.
In a nut shell yes.... but, I have never been one to view a nut shell the same way twice.
A broader picture is needed to determine if this fantasy has any other meaning beyond the physical. I am assuming that the fantasies are of a sexual nature..... A fantasy involving someone other than your partner can mean many things. The most common would be simple physical and perhaps emotional contact that is non existent in the current relationship. It is the extent to which a fantasy encroaches upon reality the determines infidelity. For example....
Safe Zone...
A Man/Woman might have regular fantasies about Angelina Jolie or Hugh Jackman because well...she's smok'n hot !!! and has attitude to boot and Hugh.. I'll just say he's a handsome man and leave it at that. His/her partner may be quite attractive and fulfill his/her needs but, a high profile celebrity like AJ or HJ sparks something within him/her that is exciting but most certainly out of reach. He/she is free to fantasize any way he/she chooses without fear of actually having to commit. No direct intention to be unfaithful.... a safe zone.
Amber Zone
A Man or Woman might have fantasies about someone close by, a co-worker, a neighbor, someone they see on the street everyday but have never met and of course.... old Loves. These involve people close at hand, people to which you would have or had regular contact with. The emotional connections are stronger and thus much more personal. This is the amber zone. When a fantasy has the means and opportunity to become reality one must tread lightly. These fantasies are more dangerous because of both the contact and the emotional. Temptation is a very strong force. When temptation is fueled by emotion very few can deny it.
RED Zone
When a fantasy, as describe above, steps from your thoughts into reality and pursuit then you have already crossed the line. The red zone. No matter the outcome you have already taken a fantasy and attempted to make it reality. The conscious decision to pursue has left you vulnerable in so many ways. On a rare occasion the fantasy may be very close to the reality...on a very rare occasion. Either way someone will always get hurt. The two may end up living happily ever after but, the transition from fantasy to reality has already caused pain and suffering to the partner and if person in pursuit has any kind of conscience at all they should feel the pain as well. Potentially opening the door for mistrust and the inevitable thoughts of.... they did it with me why would I believe they wouldn't do it with someone else?????
Fantasies are awesome!!! They allow us to experience many things we would never do in reality and a really good fantasy can feel like reality. That's a whole other psychological discussion :) They make us feel good about ourselves and have a healthy effect on our view of life if kept where they belong... in our thoughts.
I hope that was of some minuscule assistance and a very big WELCOME TO BBA Arlene :) Enjoy your stay, there are many great people here.