I was having the same fantasy for many years, so few years ago my wife and I started playing a game, where we would imagine we are fucking someone else during sex and then tell the other person afterwords. I realised that i was more turned on when I heard her choices, I used to imagine I was fucking like celebrities, but she was imagining fucking our friends. So at get togethers I asked her to wear revealing clothes etc. it grew from there, finally I persuaded her to ...... with a guy who she would commonly fantasize fucking, and I enjoy having her do that.