FF, I think that is an excellent boundary for you.
I'm glad that your wife 'understands' it too.
Her fears may make that understanding fly out the window. 20 years of history say it will.

Remember that your boundaries about living within your values, and protecting yourself from things that are against your values. Your wife's agreement and understanding aren't relevant to your enforcement of them.
When I read your description, I see not one value/boundary pair, but two of them, intertwined. Perhaps examining them will help you. Or perhaps you don't see them this way.
1. You value fidelity and living up to your marriage vows.
This absolutely means you do not ACT sexually with women besides your wife.
Not indulging in your own thoughts about other women being sexually attractive fits this value too.
I believe that your feelings are yours, and they are valid, and they are never WRONG. If you never even find another woman to be sexually attractive, that is your experience. No problem there. You may that a woman is attractive... .or conversely notice that a woman is unattractive, or less attractive than your wife. That feeling is still yours, and still valid.
What I understand about being human is that it is normal to feel attraction to other people, even when we are committed to one person, and will never ACT on that attraction. I feel attraction. I notice. I also know it is against my values/code of good behavior to stare, obsess, or cheat!
2. You value peace and kindness.
Any discussions of your desire for other women with your wife go against this. She goes into dysregulation ... .almost immediately.
She feels all kind of distress over it.
She inflicts all kinds of crap on you when she does. (You spent weeks upset over this picture!)
A boundary of protecting both of you and your wife from those interactions by refusing to discuss your sexual attraction to other women fits this value.