When I was young, I always showered with my dad. I don't remember anything that happened, but I didn't think there was anything wrong. Mom later suggested to my Dad that I take a shower with him to save some time (we only had 1 bathroom). He got kinda embarrassed and said no. That is when I realized that it had been many years since I had taken a shower with him. I also thought there was something really wrong with being naked around him.
I joined a local gym when I was in high school. It was a very small gym. They had locker rooms but no showers.. I remember my Dad joining me a few times. I saw him naked in the locker room when we would change clothes. He was a little longer than I was, but I was definitely thicker. And while I am circumcised, he was uncut. That was the weirdest thing for me, up until then, I had never seen an uncut dick.