Just once.
In my pre/teens, only once - in a mixed class, boys & girls, our 40ish apparently straight male married blackbelt instructor pinned a teen boy during a demonstration and couldn't resist making a point, tickling him briefly - The hold was certainly effective...
I remember the poor kid was ticklish, laughing & squirming, I'm sure, for a few eternal seconds.
The instructor was/is otherwise a really nice man, always careful, gentle, teaching nonviolence except for self-defense. (But if you're pinned & tickled, I say no holds barred, whatever you need to do *counts as self-defense, if you can move at all...)
There was absolutely nothing sexual about it, "just" a quick tickle... but I was completely mesmerized & horrified, thinking that boy must've been so embarrassed, I'd have "died."
I also felt pretty safe, knowing our instructor would most likely never do that to a pre/teen girl as that might not look so good.
Today he probably wouldn't do that at all. At the time I'm sure he thought a few tickles were safe on a male teen, but that did surprise me. Fascinated & horrified, especially as a pre/teen in a public class. I'd also always wondered if that happened more often.
Which is also why I've always been wary of martial artists & wrestlers...