Fantasies of Nature and Nurture

When I fantasize, I often let my fantasies “run free”. “Run free” is the only term I can think of to describe what actually happens. My fantasies seem to take on a life all their own that frequently leaves me pondering their intentions, which regularly seem to contradict my own.
I don't think letting them run free is irresponsible in and of itself, so long as they are confined to my own experience and not imposed upon others (i.e. acted out). In fact, I often think that I learn things from my fantasies, about myself, and about sexual instinct in general.
For example, when I was living as a female in prison, I was taken by surprize once by a fantasy that imagined I could become pregnant. This seemed as unsexy as anything to my mind (I had no psychological association that I was aware of that connected “getting pregnant” to the sexual pleasure of letting a man make love to me anally), and yet when the fantasy came to mind (while I was being made love to by a caring man) my arousal level unexpectedly skyrocketted! Suddenly I wanted to passionately whisper to the man who was inside my body, “Yes! Yes! Give me your babies!”
Was this mere perversion? Or, was it some sort of instinctual response to sex that lives inside all of us, not just “Jennies” (genetic females). Well, judging by the unexpected intensity that the mere idea of being made pregnant by a man I cared deeply for (and hense, wanted to “keep” him in my life) caused me to feel, I'd say it was something that came from nature, not nurture at all.
There are many other things I have learned about myself mostly, but also about being human in general, by letting my fantasies “run free”, and just paying close attention to what they do.