This happened to me last year on Kauai. My family and I were taking care of an estate on the north east shore of the island. It's a beautifully lush property with a modest stilt house. The top is considered the master home and the bottom has two guest suites divided by a shared bathroom. The bathroom had a shower, sink and toilet all in one room with a drain in the middle of the floor. Beside the driveway about 20 feet from the house is a gazebo that is walled with mosquito netting and has a showerhead and some benches around it. I think it was home once to the hot tub, but that's no longer there. The owner of the place is a good friend of ours and when he's away on business or to visit family on the mainland, often asks us to keep on eye on the place. He has no problem with us living there and having friends over while he's away. We tried to stick to one of the guest suites and had the friends stay in the other to keep the main house clean.
We have friends on the other side that have a daughter the same age as ours, so we invited them to come over to stay for three days and play on the beach across the road from the estate. Only Julie and her daughter could make it, as Matt had to work. I forget which of the three days this happened, but one afternoon after coming home from the beach, we went into the gazebo to rinse off before going into the house. Julie, my wife Irene, and I are very open, and I would've taken off my trunks to get a better rinse, but Julie's daughter is five and I'm not very comfortable getting naked in front of other people's children. I got the kids dried off, and they took off for the garden to pick papaya and bananas, and then I started to dry off while discussing dinner plans with my wife and Julie.
During this time my wife and Julie were sharing the showerhead. They were sticking their butts into the spray, opening their suits, trying to get the sand from out of all their crevasses, when Julie suddenly hooked her thumbs under her waistband and dropped her swim bottoms to the floor saying that it would be a lot easier to get the sand out without them in the way. My wife agreed and quickly got rid of her swimsuit as well. I don't know which was sexier, Julie's trim mound of red hair over her smooth vulva or the surprised look she gave my wife when she saw her shaved pubis and vulva. She couldn't stop glancing at it, though you could tell she was a little embarrassed.
This unexpected show shot a bolt through my groin and though I was only semi-hard, it felt like I was going to cum in two seconds. I haven't felt like that since high school. I left the gazebo and headed for the lower bathroom, quickly unsnapping my shorts to relieve the pressure on my aching cock. I wanted relief bad and thought that I would cum as soon as the water of the shower hit the head of my cock. We always enter the suites through the bathroom and I wanted to cum before Julie came in. As it turned out, it took a little longer than I thought and right as I was unloading four to five ropes of cum against the wall, in one of the most intense orgasms of my life, Julie slides the screen door open and walks in. She smiled at me and said: 'I was wondering why you bolted for the house so fast.' I used the showerhead to clean myself and the wall off, grabbed a towel and headed for our suite. The door to Julie's suite was open when I passed and I glanced in to see her standing facing away from me with one foot up on the bed and her fingers rubbing deep into her labia. Her hips were riding her fingers and her knees buckled slightly as she orgasmed with a quiet moan. During dinner there was a thick fog of sexual tension. That night, my wife and I fucked like banshees. I knew Julie could hear us and I could picture her cumming with us riding the waves of pleasure her fingers gave her that afternoon.