That's what she says, and she may even believe it. In rare cases, it may even be true. But IMO, women say this because they think it's what they're supposed to say and feel. She may have all kinds of different fears around this. Starting with the fear that you are just testing her, or that you will feel differently if she were to do it for real. Truth be told, there is a very real possibility that you WILL feel differently if it happens for real, or even if she really gets into the fantasy and seems to be enjoying it. You may be shaking your head, "No way, not me!" I'm just telling you...don't be surprised if that happens, as it has happened to many others, catching them by surprise.
Wives also fear that there is a hidden agenda, that what you really want is to fuck other women. Once she fucks someone, it would be only fair that you also get to fuck someone. You telling her that is not the case will not immediately convince her.
Many women have an instinctive fear of being considered a slut. For their friends to think of them that way would be bad, but for her beloved husband to think that about her? Horrifying. Sometimes women embrace that, but it's pretty rare, in my experience.
She can fear getting found out, shamed, pregnant or catching an STD. Those are all valid fears actually. Those things can happen. Don't try to argue with her. Just be understanding of any fears she expresses. Feeling understood, she is in a better position to feel safe and to work through them on her own
Accept your wife's refusal at face value. Let her feel safe, knowing that she can dip her toe in the water with the fantasy without the fear that you'll push her into the deep end. It will take time, most likely years. Let her go at her own pace. Along the way, she'll see how hard you get and how hot you are when she indulges the fantasy, and that you don't then start pushing her to go further. If she says, "You know this is just fantasy right? It will never happen, so don't even dream about it" tell her that you understand.
Enjoy the journey. Give it time, and enjoy the fun along the way, wherever it leads. When it does finally happen, it will be all the hotter for it not being something she agreed to readily. No matter how much you are sure you would love that, there's a good chance that if she jumped at the offer, you could end up finding it disquieting.
I love the jealousy level there. It's just not what my wife would not be willing to do. I think I might be chasing a ghost when it comes to hitting those hot jealousy levels with a wife that wants nothing of it....