Confessing (sort Of) Dream To Girlfriend

I've confessed previously that I want to have a Bi 3some with my girlfriend.
To sum up:
Since I hit 40, ive started to become attracted to men and she-males in addition to women, and Ive been getting into gay-bi-trans porn. My girlfriend is liberal about sexuality and has her 'naughty' side, but has a monogamous attitude toward sex for herself-us, stating when we first got together that she's not comfortable with me sleeping with other women. I don't have any specific interest in fucking other women because im happy with her. She knows i like anal sex (giving and receiving), and has an idea but doesn't know the extent of my recent interest in men, etc.
Ive been really interested in the idea of a bi 3some. She doesn't want me fucking other girls, but in the context of having a 3some, im perfectly fine with her fucking and sucking said guy with me.
Kind of a fib, but I was thinking about telling her I had a dream about both of us fucking a guy to gauge her reaction. Like I said, i would love to make it happen, but i will not risk our relationship.
Anyone out there have experience? Did it ruin the relationship, did it open a door door of intimacy and make the bond closer?
Advise welcome, especially from women.
Thanks.