I selected the male-male category but my experiences also involve a female.
I grew up in a nudist family. When I was little, pre-puberty, it was just a natural thing to see my parents, my twin brother, and my year younger sister, all nude around the house. I didn't connect any sexual feelings to the nudity as my mind didn't even work that way at that age. I don't remember ever seeing my father touch his cock or if he ever had a hard on. He probably did but at that point, it didn't register on me, I guess. But by the time my brother Jack and I were about twelve, and my sister Diane, was eleven, things began to change. We began to have erections, or at least partial erections from time to time. I guess my dad noticed it and maybe he decided it was time for us to know more about ourselves sexually. It may have been a little early for Diane but I suppose my parents figured that it wouldn't hurt her to be exposed to the sexual side of life in a family environment.
My parents had both Jack and me circumcised the same way. I have heard this referred to as a German cut and in a way, it provides the best of both worlds. I remember that my brother and I did know something about masturbation. One of the other kids in school had told us about it and one other time another older kid took me and a couple of my friends into some bushes with the promise to show us something really neat. He pulled out his hard prick and started stroking it but my friends and I got scared and took off.
One day Dad explained what it was all about and he said that we were too young to ejaculate but would be able to soon. He said that it would probably happen sooner if we jacked off regularly and that if we didn't, we would probably have wet dreams where we would ejaculate in our sleep. Naturally we liked the idea of hastening our ejaculations by jacking off frequently. He told us that it was ok to jack off any time we felt like it. Obviously we had long ago got the lecture that anything regarding nudity or sexual stuff was never to be discussed with anyone outside of our family.
The next few years were a lot of fun. It was only a short time before both Jack and I were ejaculating. As we got older and hornier we were jacking off two or three times a day. I will admit that I got a lot more enjoyment out of my orgasms if someone was there to see me do it. By the time Jack and I were fourteen and Diane was thirteen there was a lot of cum being spurted in that house. At first Diane was very hesitant to do anything in front of us but she did love to watch us jerk our pricks and shoot our cum. As she got older, she couldn't help but touch herself when she watched us cum and pretty soon she was masturbating with us. Jack and I used to jerk each other off while Diane watched. We never did that in front of our parents but they probably wouldn't have cared. My brother and I would stand facing one another and place our erect penises together. Diane would drench our cocks in a lubricant that dad gave us, and wrap both her hands around our two slippery cocks and jack us off until we came. Frequently we would both cum at the same time and we would drench her little hands with our hot semen.
Dad had told us that the masturbation experience could be heightened by lubing up a finger and slipping it up our ass. Sometimes Jack would lie back with his legs propped up and he would jack his cock while I finger fucked his ass until he came and then he would do it to me. Eventually we got Diane to finger our asses while we jacked off but at first she thought it was 'icky' but when she saw how much pleasure it gave us, she got into it with enthusiasm. One time Jack and I laid side by side and jacked our cocks while Diane was sliding a finger in each of our asses at the same time. For some reason, Diane's favorite way to jack us off was to have us get on our hands and knees with our butts facing her. She would be behind us and reach through our legs, grasp our erect pricks, and pull them almost straight down and jerk both our cocks at the same time, one in each hand, just like she was milking a cow. For some reason, this particularly excited both Jack and me and we would be shooting our cum in no time.
Eventually we would take turns jilling Diane's pussy and clit. She was one of those girls who got very, very wet and when she came she actually gushed out a lot of fluid. By the time my brother and I were sixteen and Diane was fifteen we started dating others and our sex play at home tapered off quite a bit. Jack and I would still jack each other's cocks once in a while but Diane pretty much dropped out of our little sex games completely. Looking back I wouldn't have traded that period of my life for anything and I know Jack and Diane feel the same way.
We have all grown up to be productive and solid citizens so I do not feel that our unorthodox early training in sexual matters was anything but a good thing. Having loving and caring parents certainly was an important ingredient. There was never any sexual contact or even any remotely improper touching between my parents and any of us kids. They just wanted us to grow up with a healthy attitude about sex, something that unfortunately many people have missed out on.