It happened on holiday in Gran Canaria. We had gone as usual onto the playa del ingles beach, walked about 1/2 mile to the naturist bit, where it get quieter.  We pitched our wind shelter up on top of this massive sand dune and stowed our provisions inside.
The sun was lovely and warm, no one around, laid out for serious sunbather day.
Lunchtime, gets the bread cakes (spanish crusty rolls) cheese, toms and crisps out.  hubby was still laid out whilst i started making the sarnies.  A young couple were putting their towels down just beside our dune. I smiled but no response, maybe they were shy cos I had nowt on.  I'm cutting up tomatoes and slicing the cheese. The couple don't take their clothes off beyond shorts and her bikini, yes deffo shy. Asks hubby how many sarnies he wants, gets a grunt in reply. The young couple are snogging. I've made two sarnies. Hubby gets sitting we eat the sarnies. they are still snogging. Hubby lays back down, I'm making another sarny. The fellas hands are everywhere, should I look away. I'm sat eating my sarny, the couple are having sex now, a man walks very closely past the copulating couple, he grins at me, he's walking away but keeps turning to look back around. I told hubby when things were getting to the 'I think they're gonna do it' stage, he looked once and laid back down. I was transfixed, just couldn't believe it. The fella withdrew and ejaculated on the sand.  They got up, put their t shirts on, folded their towels and left. They had just been there for that.