To be excited by the idea of your wife (or any other person) in intimacy with another is quite normal and common. There is nothing you need do or not do with regards to this, but here are some things to consider.
Would you like to talk with your wife about this and find out particulars if she wishes to share them, or would you prefer the imaginative thoughts you have already had to be all that you wish to have? If she does wish to share them, are you prepared for hearing about the reality of her sexual love making rather than your current fantasy? Often, imagination is much more glamourous than reality, so there is value in keeping this in your imagination. But as you will see repeatedly on this forum, communication between spouses is highly encouraged, although some may recognise that this is also a cultural value and may not be as applicable in non-western cultures. Ultimately you will know the answer that is in your best interest.
Consider contemplating more about the 'strange kind of excitement' of which you write. Take a moment to identify in which part or parts of the body you feel this excitement. Some of us notice that excitement is concentrated in the different energy centres of the body (chakras) and we actively harness that energy for meditative practice either when with another, or alone.
Many people here also say that poly living gives them an opportunity to learn about themselves and eventually understand themselves a lot better than they did previously. As with any set of life experiences, sometimes this is during a crisis point. At other times, it is during a joyous point. Sometimes it is in a time of non-crisis confusion, as you have expressed.
So, sit quietly with your confusion at a time that you feel it strongly, and non-judgmentally ask yourself any question you can think of about your feelings. For each question, let whatever answer you come up with rise into your consciousness, then ask yourself about that answer. Two questions to get into a pattern of asking are: "what does this mean to me?" and, "if this is true why would it be so bad?" Do not feel you need to answer these questions about the now, sometimes the natural answers lead us back to our past. Seek to understand, not judge, yourself.
Best regards
Evie