As much as I can remember about the start of my fantasies, it's been a looonnnng time since I started using them, they do have their roots in the actual sex acts I did as a child - as we would expect.
As a child I did everything two males can do, well nearly everything anyway. And I was used by two or older boys at a time frequently.
So my experience could be said to be far reaching, but my fantasies were 90% about me performing oral sex. Anal sex, masturbating, kissing and all the other details rarely enterd the fantasy, even though they were as common to me as a kid.
I think the reason oral sex became my fixation was the fact that my abusers gave me special praise for this.
I was told that I was good at it, the best, they said.
And I can't remember any other praise from those years. The teachers ruled by criticism and fear, I was an average student who's term reports got steadily worse - so my holidays were marred by criticism from my parents. I had no positive influences at all, no praise other than for giving great blow jobs.
I believed that until I was nearly 50, it was my fantasy, and then my reality when I acted out.
No prizes for guessing why ?