I think maybe my case it can be case study because I was in prudish family and there is only another man in my family is my dad who wasn't at home all the times when I was young. He was military and back home only few weekend per month. He took me out for fishing sometimes but never been nude, when he changed clothes he always put the sarong on. I think the first nude person I saw in my life that made I curious about men body was my neighbor, he was my dad's friend and was military also. As we lived in military flat, then our family were close to each others and I came to his house many times to play with his son but that family was really open to his son, they showered together even his son (same age with me) not infant anymore. One day I was in his house but his dad didn't know and I thought about go pee and walked to toilet the same time his dad walked off the toilet, he had rope on his shoulder and let the penis hung on. so that was the first time I saw adult's penis and I was shocked because I never seen that before and never known that penis could be big like that (I thought his about 6 inch but at the boy's eyes could be exaggerated)
That situation made me wonder about my body because I wasn't brave enough to talk to dad about that and I found out it by myself by searching book about human's body in the school's library and when I grew up more just talked with my friend and my nephew.
So this year I had an opportunity to get close to dad because he has to go to Bangkok to meet doctor almost every month (we live different city now) so he has to overnight with me and one day he saw my penis and I saw his when stopped by the road and peed, he asked me that night about my penis because I circumsided my penis without let other people know and he thought it was strange for him because he never known about circumcision. So after that he overnighted with me and I started wearing only my brief or took off my clothes while changing (not change in toilet anymore) I wanted to see his reaction, the first time he felt strange I known that but after 2 or 3 times he didn't care, I find my dad is quite a shy person different than me who love to take off clothes. then another time I went next step by taking off clothes and walked to bathroom naked while my dad watching tv and I stopped by the front of tv talking with him about the show.
Then when he stay with me, I find him more comfortable naked when changing clothes or go to bathroom like me and I once joke him that he has big dick ! he just smile