In my opinion it is the same reason it becomes so hot for me to watch. For me, it is watching another man take what is mine and doing things to her that I could or would never be able to. Hearing her make sounds and react like she never would with me. This all appeals to my latent submissive side which actually enjoys and craves that humiliation that comes along with that. SO I imagine you are on the other side of that equation, where it appeals to your dominant side to take what is not yours and do whatever you want with her, right in front of me, knowing she enjoys it so much more and I am helpless to do anything about it.