Normal is a relative term, usually meaning something on the order of 'what the majority does'. However that definition isn't accurate when it comes to figuring out if one is within a range of healthy experiences. We refrain from using 'normal' to classify self simply because there is a very wide range of human experiences which are healthy, which not everyone nor the majority experiences.
While typically the majority wouldn't do this your experience does fall within nominal parameters for your species (I assume you are homo sapience, if not my bad!)
I'm pretty certain that Jung or Freud would have a long intense explanation for what is happening. It boils down to you are going through the 5 stages of grief (
The Kübler-Ross model) and what you are doing is either acting in denial, or you are bargaining with the situation or combination of both.
I would say bargaining because psychologically you could, in theory, keep your mate while appeasing his desire/need for novelty. Thus this fantasy allows you to have your cake and eat it two, its a form of bargaining with the idea of your mate. This may actually be your first steps toward making such an offer to your mate.
The stages of the The Kübler-Ross model can be very subjective and internalized, for instance denial doesn't mean you will stand there telling someone 'this isn't happening' however you could build up a whole internal structure to emotionally and intellectually deny something. Same applies to the rest of the stages.
Fantasies are perfectly OK to have, even horrible ones like the fantasy about killing ones mate. Fantasy gives humans a way to work through emotions without actually acting on them. So your fantasy is OK to have, as long as you don't start acting on it and undermining your set of ethics or morals. Compromise of ones code of honor/ethics/morals tends to do a lot of damage to ones self.