Couple of times. First time, I was out for a run during college on a super hot, fall day with a female running buddy of mine from the cross country team and got super horny. I mean, it was late September in New Brunswick, really warm and nice, I was 19, she was, well, cute but not really doable cause she was, well, really skinny. But I was totally horny. We made it to the edge of town, ran up this big hill and I stopped at this cemetary and scooted over the the far end and told her I just had to beat off and relieve some pressure. Really, I was running with an erection and I was gonna burst. I kicked off the shoes, dropped shorts and shirt and settled down on the grass next to a headstone and started wanking. Holy fuck it felt good! I was never so horny in my life. And, I didn't care if she stayed or ran away...but she stayed, joined in..two of us buck naked, looking over the town, warm sunny day, masturbating like mad watching each other. I came first, fucking huge. Best orgasm I EVER had. We never screwed, don't know why, but we did whack off together a bunch of times during college.
Next time, 13 or so years later, was driving back from vacation with the wife and was a tad hung over and sleep deprived and hadn't had any poon for a couple of days since we were with a big crowd. Had a lot of buildup. Got the urge to toss while the wife was driving down the TCH at 60miles an hour. Dropped my pants and started pulling. Wife was shocked. Felt really, really, really good. Nearly splattered all over the windshield. Nobody driving past noticed, I don't think. 2nd best orgasm of my life.