Bisex fantasy - I have one
The fantasy would be to be at a swingers party, and they have an 'anything goes' room.
I would be in there with my fantasy version of my wife, and it would be pitch black, cannot see or be seen.
While I was on top of her, doing classic missionary, I'd hear a deep masculine whisper in my ear of 'relax' and would feel my back, then bottom, then hole being touched.
Eventually I am the meat in the middle of the sandwich. He would be thick, large balls, a bit forceful. I imaging the feeling of being filled near the limit would give hot feelings of pleasure and pain for me as his thrusts into me had me forced to my limit into her, ...and all 3 of us: wife, me, and mystery masculine man would collapse in a messy fluid bonded orgasm (because in my fantasy no diseases possible, everyone is clean).
:D