Just let a pre-op transexual chime in for a minute- I think you'd better let this stay a fantasy. As mentioned, pre-op is a transitory state. Most of the many, many girls I know are at their very least sexually active of their lives. Even those who were active as males have put that aside...waiting. The porn industry has manufactured an all new super-nympho that does not really exist. We are dealing with facial hair, legal matters, and coping with the muggles. Doesn't leave much time for being a sex crazed porn star.
You love your fiance. Do not tell her about fantasies that have very little chance of ever turning into anything else. Everyone has fantasies.