Fantasy is not desire

I often fantasize about sexual adventures and pushing new boundaries. I love watching gay and shemale porn. But so many times when the opportunity has arisen to have a MMF 3way with a bi couple, or suck off a CD, I just don't find myself aroused. I love tight pink pussy, but put it in my face and I'm not hungry anymore :(
Why does the approximation of a fantasy never approximate desire in reality? I wonder if Ana Mancini and Mariana Cordoba were both naked in my bed whether I would join in. My mind say an emphatic- FUCK YEAH! But my sensibilities tell me I would need half a bottle of scotch to work up the nerve (and if I came, I would just feel embarrassed).
Does anyone else feel this way?