Same thing happened to me. I could not cum or stay hard if I didn't think about her sucking a big cock while I watch. It became a "huge" problem for me. She knew it also. I didn't tell her but she knew. Even though we talked about her fucking and sucking her co worker with a 10 inch dick. I wanted her to suck and jerk him off at work.
But it began to get in the way of a normal lifestyle for me. All I thought about was her sucking big cocks. I had this big cock fascination. I wanted to suck cock with her. But, I didn't tell her, but I think she knew. Things didn't work out for us and I regret not being able to have sex with her and make her cum unless I thought about her with a 10 inch dick in her mouth.
She got turned off me always talking about her sucking other guys. She needed more love from me and I didn't have it for her. Mind you she was the biggest flit on the planet, she needed attention from every guy. It used to drive me nuts. So I finally got to the point to where I just went along with it knowing I could not give her all she needed. So, I guess it back fired on us both.
So be careful of this fantasy. It can backfire on you.