I really don't fantasize about others when I'm having sex with my husband. Even when I masturbate, if I need something to push me over the edge, it's usually thoughts of what I have done or want to do with my husband. HE is my favorite fantasy.
There is very little that goes through my mind when I'm having sex other than what is going on right then and there at the present moment. The physical sensations, the emotions, the sounds, tastes, all of my senses take over my mind and I'm drawn deep into my own little world.
I don't know if this would count, but, for the past week, my g/f has been staying with us and I will admit to thoughts of her crossing my mind while my husband was fucking or eating me, but, she was still there with us, maybe just taking a break, watching us, or in the bathroom, so I don't think that really counts. They were visions of what we'd been doing for the past few days together...either just us two girls busy with eachother, her with my husband, or the three of us entangled together.