When I was younger I only ever saw my dad naked once, he was in the bath having a soak but I really needed to pee, so I shouted at him through the closed bathroom door that I had to go, he said to me it's ok, you can come in and use the toilet. All I remember from that occasion was how much hair he had, chest and stomach full with dark thick hair, and his member just bobbing around in the water. Of course it looked huge but at that point in time any grown adult guy would of looked that way.
It wasn't until I hit my late teens that I saw him again. We were on a family holiday (worst one ever because my mum and dad weren't in a good place so they booked separate rooms) and we had agreed to go down to the pool for the afternoon so I went over to my dads room to get him to hurry up, when I arrived at his room, I assume he'd just got out of the shower as he had his towel wrapped around his waist. I said "come on lets go" and without any real hesitation he dropped his towel from his waist and went over to the drawers to grab his swim shorts. All I could do was stand there frozen, he was certainly a lucky guy, easily in the 5-6 inches range, on the thick side too, with a big set of nuts to boot and I just thought to myself, this is a man haha - It's safe to say that the only thing I inherited from my dad is the body hair and the fact that both he and I are uncut.
For those people who might find father & son nudity weird or just a down right no no, you guys need to relax, there is always instances of this happening, wether it be at the gym showers after working out together, walking in to a bathroom, bedroom at the wrong time etc, or even one day you might need to actually care for your dad in his old age.