It is always possible, but do you really want to be with a woman that would cheat on you? Now I'm being serious because some guys are turned on by their woman cheating on them. From my readings these relationships have a very limited shelf life before their woman is off permanently with a new guy.
My suggestion is to bring it up in conversation as a probe, such as an article on cuckold, stag/vixen increasing in popularity (which is very true) then ask her what do you think of that type of relationship.
Alternatively, I think from one thing you mentioned in your post you have a girl who has high emotional and relationship intelligence; she gave you the ability to track her so she is being transparent with nothing to hide to reassure you so you won't get jealous.
So if I'm right about that, my recommended course of action (which could be combined with the probe conersation) is to be direct with her during a time where you know you will have enough time to talk it out completely once you drop "the bomb" (I'm kidding because there are worse sexual desires to be insecure about). A lazy Sunday morning drinking coffee reading the paper together is an example if you do that. You don't want to not have the time to go through it thoroughly then leaving the subject hanging.
Also think through what you are going to say in what order etc. Plan the conversation. Do not wing it expecting good results. I suggest starting off with a phrase like "I have something i want to tell you about myself" then go on to express your feelings of apprehension of telling her for fear of being rejected by her. Then say, honestly I get from your post, how important she is to you which is where that fear comes from. Then be straight with her about your particular desire. If you know how or suspect why you developed this particular desire tell her.
Most people don't know where their particular kink "imprinted". It is typically some event(s) in your childhood. It took me maybe 20 years for me to come up with suspected events related to how I probably came to my voyeur stag/vixen variation (I was curious about sex, but afraid to get in trouble from a stern father so I always recruited a friend to join me with a girl since trouble enjoys company. This started for me very young checking out the neighborhood girls anatomy and was reinforced when my first orgasm with a girl in my teen years was a threesome blow job.). If you don't know tell her that you don't know why it turns you on.
One very important note. Make it clear this is not about trying to swap her getting guys for you getting girls if that is true. It is hard for someone that has standard exclusive monogamous relationship boundaries to grasp how someone could be turned on by them "cheating". They then discard this "implausible" reasoning and become suspicious, coming up quickly with "more plausible motivations" for you asking. Such as, you want to use it as cover to see other women and replace them.
You want her to come away from your intro with as clear an understanding as is possible that this turns you on and you absolutely have no ulterior motives. Then describe a particular scenario she has starred in your thoughts that you have masturbated to thinking of her. You could lead this off with a statement like "I know it is probably difficult for you to understand how something like this could turn anyone on but for instance a fantasy I recently mastubated to was".. then tell her.
Then ask her what she is thinking about what you just told her. THEN SHUT UP AND LISTEN CLOSELY TO HER ANSWER (DO NOT INTERRUPT HER UNLESS IT IS TO ANSWER HER QUESTIONS).
Very important here to really concentrate on her words and body language to understand her feelings as best you can. Too many guys don't understand for most women this is a process of a minimum of days, to weeks, to months to never. They think if they keep talking a sales pitch it will turn the tide. It will not.
Alot of women, if not most, have a vision of marriage that isn't what you are describing to her. So it is going to take time for her to create a new vision of your marriage, which if she really loves you she will try to understand or at least appreciate the courage it took for you to broach the subject with her, even if she isn't initially interested in entertaining your desires. I say initially because there is a high probability that sometime down the road a situation will present itself (a hot guy from school she wanted that pops up in your lives) that she will want to explore so now she can see the benefits of being with you.
Now you could also be surprised and she could be all in and tell you how hot it sounds. This is a pretty common outcome from my experience.
Be honest and sincere with her and your own emotions around the subject, the latter i have found to be more difficult. You will learn alot about the strength and variables of your relationship. I have had this conversation with multiple women in my life. I never had one run screaming from the room. Most loved the idea as they start thinking about all the possibilities.
Then if she seems not totally disgusted (always a possibility, but very rare), ask if you can do some fantasy talk during sex of your fantasy. If you have a relationship that is on a good foundation I predict you will get to some variation of what you want eventually. If it turns out your relationship isn't as strong as you believed, it is always better to learn that earlier rather than later. If her reaction is bad you will have to make a decision on if you can live without your sexual desire. It could change her feelings too, but I think that is extremely unlikely unless she has dogmatic religious beliefs (I have been with a number of conservative church girls doing 3somes and 4somes).
Good luck. Keep us posted on your progress. Lots of experience on this board to help you in your journey.
