That really is a good question, but sadly I can't seem to be sure which one it was. I assume it was this one, and it needs to be mentioned I already knew I was into this stuff when it happened and was reading the stories on here sometimes.
A few years ago in spring I jogged to a lake in the morning hours and spontaneously decided to take a swim. I looked for one of the many little coves that are directly besides the path around the lake, took my clothes off and enjoyed the freshness of the water. When I came out again I stood in the sun, with my back to one side of the cove, the lake one meter two my right and the path two meters to my left. After a little while I started to dry myself with my t-shirt since I had no towel. Just few seconds into drying myself I noticed a women swimming along the shore, directly towards me. She was swimming calmly, taking her time and looking directly at me. Just a moment after I noticed her I noticed some movements behind the trees in front of me to my left. Two girls riding two horses slowly appeared in front of the trees. They where riding in slow step speed and taking notice of me immediately. Now I was standing there totally naked with three onlookers appearing out of nowhere, just around three meters away from me and if my luck was not enough they where taking their time to pass me and where not at all too shy to look. All this happened so fast but I tried to let the feeling sink in and make the best of it. I pretty immediately started drying my hair to shake my dick a little bit with my movement, and to be able to take discreet glimpses at my three friends. They all where only looking to the front for a little part of a second, to see where they are going. The rest of the time their heads where tilted and their eyes looking at me and my slowly hardening dick. I was not hanging anymore but already full of blood and slowly rising a little bit. I was focused a little more on the two girls on their horses to my left. They where clearly interested in seeing a young guy only a few years older naked and where even more interested as my dick started getting bigger. The women two my right I felt was enjoying the whole situation, seeing me, seeing the girls enjoying to see me and seeing me enjoying all this being seen. Then I started to put on my t-shirt, taking my time when it was over my head, as I was able to see through the fabric. I was really feeling and enjoying the moment. Seconds later the two girls to my had to take their eyes off me as they were passing me and vanishing behind the the trees. I had put my t-shirt back on now and was looking to my right to see the woman now at her nearest point to me, looking at my body with her calm eyes, like she was doing the whole time. Then she was also passing me and this super lucky 30 seconds where over. Sometimes you just are in the right place at the right time.
I'm a little tired, so please excuse my writing skills.