Thanks. I think the key element for me is the jealousy. Who can explain that? In the right proportion, jealousy, for me, is the most potent aphrodisiac of all. But it's like handling dynamite - you have to be extremely careful with it or it will blow everything the fuck up.
I have two tales to tell. I had already written them up when I started reading this thread. Here's the first.
My earliest memories of being aroused by second-hand experiences all involved three second cousins of mine. All beautiful girls and all notoriously ticklish. I can't relate the details because we were all under 18 at the time. I wish I could, because they're all formative experiences, but, rules are rules.
I don't think I made the connection that the jealousy itself was the source of my arousal until an event that happened involving my 2nd wife.
We were involved in a self-help / personal growth group at the time and we were on a weekend retreat. Come to think of it, this was before we were married. This was a regularly occurring retreat so we knew many of the people there, and many of them were close friends. One of them was an especially obnoxious guy - the kind who's just enough of a prankster to actually tickle someone just for the hell of it.
The event occurred as the retreat was ending. The retreat itself was designed to be a very physical, touchy-feely kind of experience. There was a big meeting room where most of the activities occurred, in which the participants laid on the floor on blankets and sleeping bags. We were cleaning up the room, when I suddenly heard my girlfriend screeching and protesting. We all know that sound - it can only mean one thing. I turned to look and our obnoxious friend was leading a gang of three or four people - it's hard for me to remember now, but they might have all been men. They were wrestling my girlfriend to the floor. Once they had her down, the others held her while our friend started tickling her sides. She shrieked with laughter and couldn't escape. I remember, too, that she seemed to thoroughly enjoy it, which added to the intense reaction I had.
It lasted only a couple of minutes, but it made my blood boil. She knew about my fetish and was almost repentant when I told her how it made me feel. Of course, I didn't hold it against her. What surprised me was how intensely aroused it made me feel thinking about it afterwards.
Sometime soon after - it may have even been that night when we got home - we made love and I drifted into a fantasy of her being tickled by someone else. It wasn't our friend, that felt a little too weird. I think I just made someone up. The point is, the intensity of the fantasy was almost overwhelming. At that time I was too self-conscious to tell her what I was thinking about, but it was the most intense sexual experience I'd had until then.
Eventually, as I grew more comfortable with her, I did confess to her how much it turned me on to think of her being tickled by someone else. While the idea did nothing for her, she did love me and she didn't judge. We even toyed with the idea of trying to make it happen. We even got as far as privately picking out men in public who would be good candidates. I'm not sure if we would have ever actually gone through with it. Sadly, life happened and our marriage ended before we could ever go any further with it.