I think it's called a fantasy for reason..doesnt mean it HAS to come true.
Just depends on the relationship and how important that Fantasy is vs. the relationship. Will it fester until I get my way, (them or a side project) or will/can I just get over it and let go.
I cant say that I have that issue on that level. Mine are pretty much fullfilled, I have one thing I can think of but maybe I save it for a rainy day.
I generally like to have these talks before I even start with someone, if I know there is something I DEEPLY need and I can see them not meeting that need I discuss and we can decide will this work or should we say goodbye.