go on tell your wife your fantasy and make it a mutual fantasy.
Well, I can't come close to topping your story, but as far as it becoming a mutual fantasy; that won't happen in the Bear's lair. Mrs. Bear is OK with my interest in FvF, but she found out after over 3 decades of marriage. (Obviously, she's no Sherlock Holmes.) At first, she was taken aback, but rationalized that I was a red-blooded, virile guy and this was an outlet. After all, I had been a good husband, father and upstanding member of the community, so judged against that I stood up pretty well. When she asked me about it, I was prepared -- after all I had a long time to plan for the inevitable. The first thing was to reassure her. I explained that this interest didn't just pop up -- it pre-dated puberty. I also reminded her of several times in our lives when she had engaged, or nearly engaged, in physical competition with another girl / woman and how it excited me. She recalled everything, but was really surprised (and I think proud) of the effect that it had on me.
As time has gone by we've had other brief conversations on the matter, which involved me explaining my take on the proverbial "why?" question that gets posed a lot here, and challenging her that a similar competition involving me would have an erotic impact on her. So, we're at a point where she's comfortable with it. A few weeks back I told her, "I couldn't sleep last night, so I went downstairs and turned on the TV . Guess what was on -- my favorite episode of I Love Lucy." She replied, "Oh, the one in the grape vat. That's one of mine too."
Though far from being a mutual fantasy, I consider that progress.