I was 15 at the time when this happened.
My family was at a camp ground and there was only one other family there. Our families began talking with each other. My family was just me, my mother and my father. Their family was a mother, a father and they had a daughter who was about 18 or 19. I figured she would want to have nothing to do with me but she had no problem hanging out with me. She suggested we go on a hike and out we went. There was no one around until we got to this one area where we saw a couple making out. They were naked and we hid as we watched them. They didn't know we were watching and we saw them have sex. Once they were done they got dressed, walked down a path and into a car and drove away. I was very amused seeing this and the girl I was with admitted she was too. She asked me if I have ever seen a girl naked or if any girl had seen me naked. I said I had seen my mom and she said that didn't count. I told her I hadn't and she asked if I wanted to see her naked. I was surprised by her comment and told her I wouldn't mind seeing her naked. She asked if I had any sisters and I told her I didn't. She told me she didn't have any brothers and wouldn't mind seeing me naked. She suggested we both take our clothes off and see each other naked. I was reluctant because I thought she may be playing a joke on me but when she started to get undressed I knew she was serious. Soon the 2 of us were naked and checking each other out. She wasn't shy grabbing any part of me and wasn't shy about showing herself off or letting me touch her anywhere I wanted. We both seemed to be very interested about seeing what we each had between our legs. She was definitely interested in seeing my cock and I was definitely interested in seeing her pussy. She almost liked showing her pussy off me so I didn't mind her seeing and touching my cock. I don't know how long we were naked out there checking each other out but it was a while. We didn't have sex but any mysteries I had about what girls looked like naked were answered that day. Once we got dressed and went back, our parents asked if we saw anything interesting on our hike. It was a bit of a loaded question and the girl said just trees mostly.
The next morning that family was leaving but the girl wanted to shower first. Her father had begun packing their car and said he needed to get some gas and some other things and he could do that when she was showering. She said all she needed was a towel. She walked behind her car and came back wrapped in her towel and her dirty clothes in one of her hands. Her father asked what clothes she needed and she said would get them when he came back so they don't get dirty or wet in the showers. He didn't seem too concerned his daughter was just in a towel and he was going to leave her that way for a while. Both her mother and father drove away. She turned to me and told me to go down with her. My mother heard this and she said she would get me a towel and said to strip down like she did. My mother held the towel as I undressed behind it. She seemed in a playful mood because she dropped the towel on purpose a few times and yelled out "naked boy alert, naked boy alert" each time. My mom was trying to embarrass me in front of the girl but she didn't know what happened yesterday. The girl then opened her towel and flashed herself to us. Both me and my parents saw her do this. My mother thought it was funny and a big smile came over my father. The girl then grabbed my towel and pulled it away from me leaving me fully exposed. The girl was now standing with my towel in one hand and her towel in the other so she too was fully exposed. My mother was laughing and my father had an even bigger smile on his face seeing this girl exposing everything to him. She gave my towel back and we walked to the showers with just our towels in our hands. Being that no one was around she came in the men's showers and we showered together. I don't know if my father really needed a shower too or if he just came down to check on us but he showed up at the showers too but he had his towel wrapped around him. It was one big shower room so when showered you saw everyone else shower and everyone saw you. He caught us in the men's showers together and when the girl said she would go to the women's showers he said there was no need to. He dropped his towel and there he was naked showering with us. I don't think the girl minded and he definitely didn't. We finished up and back we went. Her parents were already back and she went to their car and removed her towel and began putting on her clothes. She did it in plain view of everyone including her parents and they didn't say anything. My father pulled me aside later and said that I was not to tell my mother that the 3 of us had showered naked together.
It was a fun and interesting camping trip.