Truly, I've had a handful of girlfriends and told every single one about my fetish. I have never been ridiculed, mocked, or made fun of for it.
In fact, quite the opposite, I've always been greeted with intrigue and
interest in the particulars of it. Besides, almost 100% of the time, if you tell your girlfriend casually that you're ticklish she HAS to take the bait.
It's instinct. 😀