This happened this summer at a nudist camp I have visited often. I have been to nudist camps many times and I have never had an erection though some of the sights have warranted one. This time it happened I was totally nude walking down to the beach from my car when a young woman probably in her early twenties came by me, she was topless but still had on bikini bottoms.
Anyhow we began talking and she told me it was her first time at a nudist resort and she found it exciting to be there. She was petite, with blonde hair, glasses and very cute. As we stood and talked I could feel my dick growing, I tried to keep it where she wouldn't notice, she asked about the three wheel bikes that floated on the water, I told her what they were for and asked her if she wanted to go for a ride.
She agreed and we went down to them. When we got there we lay our towels on the seats and she removed her bottoms. One look at her perfect bottom was all it took and I was rigid, my penis was straight up against my stomach. I knew she saw it and I felt embarrassed, I apologized and she just giggled and said it was ok. We rode for a while and headed back to the beach and I covered up as best I could as we walked back to the beach. When we got there we lay out our towels and I quickly laid on my stomach to hide my boner. She sat directly in front of me giving me a great view of her clean shaven pussy.
Well laying on my stomach didn't help my erection, it made it worse. The friction of the ground and towel was building up inside me. Without touching my penis with my hand I felt that sensation. I gave myself the mental ok and with that my hips started bucking and I was cumming all over my towel. When she saw it happen she asked if what she thought she saw happened I said yes, she again giggled and said that it was ok and it kinda turned her on seeing me so helpless over it.