My G/f told me about this site, and has contributed herself a couple of times, since when I have been an ardent fan. This happened to me some years ago when I was 22, I'm now in my 30s.
I was on a golfing holiday in Spain with my sister, her hubby and a golfing friend. We had rented a small (cheaper, lol) villa in the golfing complex, with two bedrooms but I was fine about sharing a room and letting my sister and her hubby have the other. I had met the guy I was sharing with through my golf club at home, and we'd known one-another for about six months by this time. He was older than me, about 38, married and with kids. He was a slight built guy, not much more than 5'2', but very fit, no trace of fat or excess weight and deceptively strong. We'd often spoken and joked about sex, in the way most guys do, and this had included some confessions about our teenage experiences, specifically an admission that we'd both wanked other boys when at school.
Our villa was the last in the row, next to the complex's swimming pool, and slightly elevated. It was just possible to see one corner of the pool from our bedroom balcony, and also to see down onto the villa below and to one side of ours. Late one night we were both sat drinking on the balcony, sister and her husband having gone to bed. It was still warm so we were both in just boxer shorts and tee-shirts, the moon was out and we had a bottle or two of wine and some bits of food to nibble at. Presently, we heard hushed female giggles and whispers from the direction of the pool. We both stayed quiet and tried to determine where the noise was coming from, and what was going on - though we both had a fairly good idea as to the latter!
Within a few moments we'd discovered the source, helped by the sound of water, and my friend had found that by standing on one of the chairs he could see the couple responsible. I was worried for his safety, as to get a better view he had to put the chair he was standing on precariously close to the edge of the balcony rail, and he had nothing to steady himself with. Nonetheless, he started giving me a whispered running commentary as to what they were doing. For my part I tried to steady the chair, but that wasn't too successful and in the end I reached up meaning to steady him by the waist. As I did so, my arm brushed across the front of his boxer shorts and I could feel his cock on my arm through the material.
I immediately realised that he was well on the way to a full erection, and despite knowing I should move my arm I hesitated a moment or too longer. He moaned slightly as my arm stayed in contact, glanced down at my arm and then me, then went back to watching and commentating. A few moments later, and I could see that he had a real hard-on, his cock was pushing the front of his boxers well out. The couple by the pool were now fucking, and we could both hear her stifled gasps/moans and comments. I was hard too, and wanted to look, but at the same time I was fascinated by the feel of his cock which by now was making frequent contact with arm under his boxers. In fact, a couple of times I thought I sensed him push slightly as if wanting more contact. Feeling really horny, I took a deep breath and let my hand slide down from his waist to the protruding part of his boxers, and touched. No adverse reaction, rather the opposite - a slight groan from him as he glanced down at my hand, and I knew he was up for it.
I managed to ease his boxers down enough to get his cock out. It was so much larger than I'd expected, and than mine, or maybe his slight build made it appear deceptively so. In any event, it was certainly very hard and thick. I started wanking him, and he whispered for me to go quicker. It was sounding by then that the couple fucking were pretty close to concluding their fun and I guess that as a turn on for him, as it was me, too. Within what seemed only a few seconds, but which was probably a couple of minutes, he said he was going to cum. I slowed the pace and he shot his cum, loads ending up on my hand and wrist. Sadly, the couple had by then finished their fun and it had gone quiet again, but I was still harbouring a hard-on. As he got off the chair he saw my bulging boxers and said that I'd better wank off now, then added that better still, he'd do it for me in return.
Rather oddly, and a little worryingly at first fearing he might have 'other' ideas, he had me stand facing the balcony while he stood behind and slightly to one side of me, then he reached around and grasped my cock and proceeded to wank me whilst looking over my side at what he was doing. He told me to tell him when I was going to cum, which I did and which didn't seem to take long at all. And as I came, he moved so that he could see better.
It was an event which we never mentioned or repeated afterwards. But which I found really horny at the time.