When I was four years old I was supposed to take a nap midmorning, but I'd outgrown naptime and was always wide awake so I'd masturbate like crazy (humping a teddie bear) while fantasizing that a group of women wearing bathing suits were passing me back and forth over icy cold water in a swimming pool. I was turned on by the fear that they would slowly lower me into cold water. That was my first sex fantasy and I masturbated to it for years. Then, sometime while approaching puberty, the fantasy switched from fear of being lowered into cold water to fear of women overpowering and tickling me which at the time was a physical possibility. The two fantasies are similar and involve loss of control and helplessness to sensory stimulation. Through my teen years I kept masturbating to fantasies about strong sexy women dominating, outwrestling, and tickling me though as I got older and started lifting weights the scenarios became physically unlikely.
One day I discovered a copy of The Joy of Sex in the closet and learned about bondage. The book even had a chapter about it with a picture of a man tied down, a woman sitting on him; and the author mentioned foot tickling: how it was torture for some but it turned on other people out of their minds. Soon my sex fantasies graduated from the less likely being outwrestled to simply being tied down. I imagined being shamed, being forced to admit I was ticklish, panicking--fearing she would never stop, and begging for mercy.
Women getting tickled turned me on too but originally it was more the idea of provoking revenge. As I've gotten older though, I've begun having more fantasies in which I'm the tickler and the woman is laughing frantically and helplessly.