Advice: GF having shame around thoughts of a threesome

I'm nb34, and shes F37, and we've been together 13 years now. Within the past year or two she's started having big anxious/shameful (her words) feelings revolving around the thought/desire of having a threesome, or engaging in sex with another trans woman. There is a lot of nuance in the things we've talked about, but I'll try to keep it concise but thorough.

Going into the relationship, I have brought up multiple times how polyamory would be off the table for me, and still is currently. I acknowledge this is due to my own insecurities and attachment style, even though I do entertain fantasies about poly arrangements or a threesome, but the risk always outweighs the potential reward.

About a year ago she came to me crying, telling me that she felt so bad about thoughts of wanting to sleep with another trans woman, feeling like she was hurting me from just thinking about it. I've done my best over the conversations weve had to assure her it's not wrong or bad for her to desire that, but that I need to know if she's going to be happy with me if I can't facilitate that. She'll say things like how she doesn't want to leave me or find someone else, or hurt me, but that she thinks the desire will always be there in some way. (I've tried to ask her like, can tolerate having the desire without being able to act on it? can she be happy with me without having sex with other people?)

I've told her how it's not off the table in the future. And in talking about things, I've tried to explain to her all of my apprehension, and all of the logistics of how we would go about this: would we break up and let her explore? would we open for specific occasions, or would we attempt being poly even though I can feel that I am not prepared for that emotionally. I've also tried to ask her to give me more information her thoughts and feelings and expectations on the matter, but it seems like every time we talk about it she's just like "i just wanted to vent, I didn't want you to try to solve this problem". (Because I'll tell her, what are we going to do about this? How can I help if I can't help by being poly?)

I've been honest about how this topic brings up fears of abandonment, cheating, fears that I'm not enough or not good enough, and worries that shes lying to me/herself about just wanting to be with a trans woman instead of me. It's rough x.x

So it's come up a handful of times over the past year, always very emotional on her end where she's often crying, she says that she's happy with me and doesn't want to leave me or find anyone else, that she just wants to experience the very specific experience of being topped by another pre-op trans woman. She'd like me to be involved, and wants it to be very casual, no feelings, no strings attached. I've told her that I also have thoughts of threesomes, maybe even more frequently than her, but the risk always seems like such a hassle and trying to find the perfect person/situation just doesn't seem worth the trouble, especially if I'm not seeking anything more than sex.

I have difficulty navigating this whole thing because it feels like it's a problem needing to be solved, and I either need to get over my insecurities and go poly/open for a threesome, she needs to let it go and accept it won't happen in the near future, or we need to end things very reluctantly so she can see what it's all about.

I guess any advice would be welcome. I'd give more context but we're all such complex humans I'm sure yall can appreciate me NOT writing a memoir here hehe.

ETA: "its not off the table in the future"