@curiousity123,
I can say as a man I've often fantasized about my wife with other man. There have been discussions but she is not comfortable. I agree with the previous poster that this should not be at the core of intimacy. It should be a nice occasional compliment. I've struggled for years communicating to my wife that I'm in to more kinky stuff than she is. I think the best thing to do is tell him you're uncomfortable with it and ask if there is a happy medium. For example, if you trust him to keep it private maybe take video of the two of you together and he can watch it later. That may be a nice compromise to satisfy his voyeurism when it comes to you.
That being said, make sure you are emotionally connected on other levels. I do a lot of introspection personally, but even I'm susceptible to incorrectly substituting sex when I need emotional intimacy elsewhere. Make sure he isn't repressing other issues in the relationship and trying to manifest that need through sex.
Lastly, the worst thing you can do in a relationship is capitulate when you are not an enthusiastic participant, and that goes double for sex. Don't do anything you don't want to do, you may feel like you're meeting his needs but it will only result in resentment. Separate what you want from the desire to be an accommodating girlfriend.
Good luck