I had often fantasized about watching my wife with another man. A hung, muscular guy. I have an average sized package and am pretty good and attentive in bed and I happily go down on my wife at any time and am secure enough to bring toys in the bedroom to help her get off.
I kept that fantasy secret for years though. While we had sex a fair bit we didn't TALK about sex very often and it seemed too weird for her tastes. One day though, i decided to give her a little 'quiz'. I said she had to write down 5 things she'd be into sexually that we hadn't done before. I said I'd do the same. Then we'd swap lists and we'd rate what we definitely wanted to do, definitely didn't want to do (no judgement, no questions asked), and what we'd consider.
I had "watching her with someone else" as a fantasy but also had the disclaimer that I didn't know if I ACTUALLY wanted it, or just wanted to fantasize about it with her.
About a week later, we came home after doing some drinking at a neighbours and had a pretty hot session. Afterwards, I brought up the list again. We both agreed that we shouldn't do it but one of the other things on my list was taking erotic photos of her (which she was open to - she's shy but she's also very photogenic and I think secretly likes to show off). I asked what if we staged a photoshoot to make it LOOK like she's with another guy. We'd find a 'fantasy guy' from a personals site and just stage this hot session but she wouldn't actually sleep with him. To my surprise, she said she'd try it as long as the photos never saw anyone's eyes but ours.
So I get her to describe a potential guy. She doesn't mention cock size but I make that my own secret requirement. We eventually find this guy and set it up. I plan to do the photography and it is all done in our own bedroom to make it look authentic, etc.
We have a quick drink and just decide to get to it. It starts out pretty awkward so we agree that they can do some making out, just to get comfortable and get him aroused.