I've never tickled any of my girlfriend's vanilla friends. I've tickled some of her/our fellow tickle friends though, but I assume that is not what you were talking about.
Why would you want to do that if you can tickle your girlfriend/wife?
Because for some of us, it can be sport only, and doesn't have to have to be sexual. And it's nice to play against another team sometimes in sports.
So, if you have an understanding wife or gf, it should be fine, as long as you keep your hardware in your pants.