I'm betting there is a fair amount of tickling that goes on in the "vanilla" world just without the emphasis and attention tickle play receives from those of us who may frequent a place like TMF. Tickling is an integral part of play and fun and intimacy between my wife and I but she has no interest in spending time on TMF or other online tickling sites (though she has looked over my shoulder when I am cruising through tickle pics and vids).
I think it is the media that often sensationalizes the more exotic tastes in things like tickling or foot fetishes. I don't recall ever seeing a movie or TV show that had a scene or a character with a preference for feet that wasn't depicted as some sort of oddball character or loser with a capital L.
And from time to time the news media picks up on a story of someone who is unable to control urges and is caught sneaking around sniffing feet or tickling when it is unwanted. Our society is built on people generally exhibiting a modicum of self control whether that be tickling a stranger or spitting in church or dressing for a day at the beach when attending a funeral, etc, etc.
For the most part, I think we who have a passion for tickling are not much different from any other segment of the population who may have a particular love for something whether it be skydiving or model railroading. Only tickling can veer into the territory of bondage and fantasies of tickle torture and, particularly in our Puritan heritage United States, combining tickling with sexual intimacy. Not all of us get turned on by tickling or being tickled but enough do. And people can get very nervous very easily when something includes sex and erotic arousal even if it really fascinates them.
Like rhiannon writes, many of us in the tickling community probably get more tangled up in what we think others perceive of us when, in fact, most "vanillas" may not give tickling a second thought (except maybe to attack their unsuspecting significant other for a bout of tickle attack/wrestrling).
