Yes, some forms of proper, socially-acceptable nudity, aren't really "sexual."
And neither should mothers needing to breastfeed in public, be considered "sexual" nor "obscene" either. An exposed breast is quite understandable and moral, if there is a baby there to explain for it. As breastfeeding in public is common in developing countries still more pronatalist than us, and people eat in public, and babies have their obvious needs.
I don't know what all the fuss is about. Your dad's just another guy. Why be embarrassed to be nude in front of him or for him to be nude in front of you? It's not a sexual thing - or at least it shouldn't be.
I've seen my dad's and he's seen mine. We swam together for years and would get changed together. Should I have gone to hide round the corner to avoid seeing him or him seeing me or should we just have carried on our conversations like two ordinary guys? If I'm playing sports with my friends, we shower together and carry on talking even though we're naked. Why should I be any different with my dad?
People find "sexual relief" from their powerful natural reproductive urges, by
reproducing, and so of course as people responsibly pair up, marry, and procreate children, the number of human penises throughout the world, continues to naturally rise. So why should it be any "shock," that a son might glimpse the penis that naturally gave rise to more penises (including his own) entering the world? Just one of those "facts of life."
A father can be a friend as well, and guys naturally can morally have a little natural nudity in locker changing areas or the communal showers. Just because one's friend happens to also be their Dad, I'm not so sure how that's supposed to change things? Well unless somebody is doing something immoral and afraid of getting caught by their parents?