I experienced that with my same sex ex-t as well and it disturbed me for a long time. I figured it was some Freudian thing -- something to do with my intense desires to please her, fill her needs. Much like a child needing to deeply please her mother in a loving way in order to be accepted which somehow translated into sex for me. I don't know. But, I eventually stopped trying to analyze it, accepted it, and came to enjoy the fantasies.