Communication and trust. My wife is conservative in everything but this, she currently has 3 boyfriends. When I told her this fantasy, she got quiet and I thought she was mad. I pretty much dropped the subject, but I did make it clear when I told her that I did not want to ...... with other women.
After we started this journey we talked about it. I had been with my wife for 25 years. She trusted me and knew I was telling the truth when I said I did not want to have sex with other women. When I told her the fantasy, she had been playing Pokémon Go with a man she had met while playing the game. She was attracted to him and after I told her, she started to look at him in a different way.
The fact that she trusted me and I did not push her, helped her. The first time she had sex with him, it was my birthday and I was at work. She told me about it at dinner in a restaurant. I thanked her for the gift and asked her if she had enjoyed herself. She said yes and I dropped it. Talking with jer later, if I had pushed her for details, it would have made her uncomfortable and she probably would have stopped.
So talk with her and make sure she trusts you.