You sound a lot like myself. Some of the stories I've been told haunts me to this day, because it pains be to think of someone I love helpless and tickled by other (probably horny) guys, but at the same time my body reacts with sexual excitement. It's actually pretty messed up that something can make you feel such conflicting feelings.
at the same time.
I was in a relationship some while ago and this girl had an uncle that was let's say "a physical joker". He kept poking her all the time and well, to be honest it was very uncomfortable for both me and her. One day at a family birthday I suddenly hear a scream coming from the garden. I immediately recognize her laughter and on instinct I just run out. There she was, pinned down on the ground on her stomach with him tickling her sides mercilessly. I still remember her desperate look to this day. He was getting her good, not the playful kind of good, but really really tickling her. Her whole being "raped" in front of me, because yes, to me tickling is sexual. I was just speechless for hours afterwards playing it over and over in my head.
It probably didn't last for more than a minute or two maxmium, but this "event" was something I needed time to get over on an emotional basis. Now, at the time I was mostly furious at that old perv and I didn't want to meet him again, in fact I don't think I ever saw him again after that.
Now, years later, this has become one of my favorite memories, although it still pains me it's also one of the most exciting things I've never witnessed. And what I once wished never happened, what I wouldn't do to experience it once again.