Two years ago, when he and I had been together for about a year, and he'd just moved down to NY for a new job, he developed a mutual crush on a little office girl. Being that I am into sharing the wealth (within guidelines) I told him to go for it; he just had to report back to me with any kinky events that transpired as a result of his pursuing.
He was commuting between MA and NY, and would usually be home Thursday through Sunday. One Thursday night we were out at dinner, catching up on the week's events, and I asked "So what'd you do this week?" He said oh, worked on some projects, bought a new kitchen table, and "Oh, and I jacked off in front of the office girl."
He was a bit hesitant to share the details, as this was his first "extracurricular" experience in the year we'd been together, but he soon realized that it was very erotic for me to hear the details of his jackoff session, so he went on to tell me:
They'd been at lunch and he told her, when we're done with lunch I want you to get up and walk out to the parking lot, get in the front seat of the car, and I'm going to get in the back seat and jack off.
Which, he did! Then, evidently, they just went back to the office and resumed their workly responsibilities.
She thought this was hot, I thought this was very hot hearing about it, and as far as I know there were no more jackoff shows for her...
...but in my imagination there were many, many more.
Second hand jackoff stories (especially the true ones) are almost as hot as live shows.