My families ranch had a few horses we would rarely ride. One time I was riding with my grandpa and grandma, as I've mentioned my family was nudist so only I wore anything which was some shoes because I was young. I remember this day especially because my grandpa never had a erection but today he had one all morning he was even joking about. He actually had a huge dick covered in thick grey pubes. I remember walking by him and he stopped me and pointed at it and said "this is why your grandma loves me" he then grabbed it by the shafts and shook it saying "Yeah, your moma came from here." He then walked off to ready the horses. I'll be honest seeing my grandpa bounce around his huge erection getting the horses ready got me hard. We werent riding far just around the ranch. I didn't ride much so grandpa rode ahead and grandma behind to make sure im not messing up. Anyway we rode for while before I just natural lagged behind, I did because I was distracted by my raging boner I had for a while and the way the saddle sat on my taint didn't help. I tried to focus on fallowing in a line behind them, my breath was shaking and my cock was throbbing I shifted a little so my balls could feel the vibration probably looked funny riding. I rode like this for a minute practically quietly crying and whimpering to cum. Just the. The horse actually flipped its hair and it landed on my tiny throbbing cock causing a rubbing sensation making me cum hands free. I didn't even clean up I just finished the ride quietly and left with my mom.