If my own experience is any indication.... before I had my first sexual experience, my fantasies were VERY unsophisticated. It wasn't until I'd fooled around with a girl for the first time, got a sense of how things worked, felt, etc., that I started to come up with more elaborate scenarios.... I could imagine, for want of a better term, set-ups (the environment we would be in, etc.), but couldn't really picture the acts themselves.