As others here have stated so well....there are many different types of tickling. We all have a conscience and know right from wrong; whats appropriate and what isn't.
When I tickle my girlfriend, is it a turn on? Most definitely....just as when I was 18 and tickling my female girlfriends. It's a TOTALLY different feeling than when I tickled one of my sisters or niece.
When I tickle my girlfriend, it's "erotic tickling". When I tickle my niece, its playful...usually just to hear her giggle, same as my mom and aunts did to my sister and I when we were kids.
Here's a vivid example of the difference:
I've known my girlfriend Traci since she was 4 yrs old; she was one of my oldest sis's playmates. In those days, when I tickled Traci, it was the exact same feeling as when I tickled Becky (sis)....either to tease or for revenge (sis and Traci both loved to tattle; get me in trouble with mom and dad). Why? because in those days, I saw Traci as another little sister....and felt that way about her even after she graduated high school (I worried myself to death about Traci when she joined the army....the same way I'd have worried about sis).
HOWEVER, since I began dating Traci after her divorce last year, it's totally different. I no longer see her as a younger sis...she's my soulmate, and when we tickle now it's not the same.....its very erotic, and turns both of us on.