Recently I started having this fantasy of watching my gf fuck another, very well hung man, in front of me.
What you are saying is a bit a turn on too. For ex. it would be a turn on if while in bed, she would tell how she felt attracted to a guy and what she would like to do with him.
The brain is the biggest sexual organ & zone and our fantasy the best sex toy. If you think I'm right and want to share some ideas, feel free to drop a PM.