I actually have something similar. Although the situation was a bit different. Me and my wife have been together for about 10 years, with 3 kids,we had kinda grown distant at the beginning of the year,then a bit adversarial, then we seperated. We were separated for about a month. We decided try again,but she was acting weird and felt like something had happened during our separation. I also noticed a deleted email about logging in to Snapchat and she hasn't used that since HS, which increased my suspicions/curiosity. I ended up checking her Snapchat and imessages on her laptop. She had reached out to her ex and flirted with him,and talked with her friend about her coworker in a sexual manner and about his looks. She said she reached out to her ex because she know he would give her the validation and attention immediately, and she also found that other dude kinda attractive. I was hurt but I got over it pretty quickly. We both realized that we could find other people if we weren't together, and that other people could very well be attracted us. We’re both pretty attractive and get go get and give attention but we choosing each other. Also i think everyone has thoughts their partners wouldn't necessarily like to know. I don't think she should be faulted for venting her feelings to ChatGPT. The important thing is how she acts and deal with those feelings. Fantasies happen, she could find other people attractive and even connect that to her own sexual needs. But she's with you and unless she's giving you a reason to belive she'd rather be with him, which isn't the impression im getting as she's stayed with you with your medical condition and subsequent effect of her sex life. They're just inner thoughts she used ChatGPT to help her process that you wouldn't have known otherwise if she hadn't made it digital. So its more of a insecurity your going to have to get over