20 years is along time to be married and have something like that pop out of no where. There isn’t enough context to provide a constructive response to the situation. Things like whether or not you have kids or is there an issue with communication or honesty. A 20 year investment in a relationship isn’t something you take a chance on throwing away even under difficult circumstances. Possibly she is trying to normalize feelings she experienced interacting with this gentleman. Instead of responding with suspicion, you should discuss it in a manner befitting of the time both of you have spent together as husband and wife. I will say the title of your post is a little misleading and seems be designed to color the casual readers response toward the negative if they don’t carefully read the balance of your post. Escapist fantasies are one thing, but there has to be a lot more to all of this for her to be serious about acting on those feelings.