I'm not sure masturbation is instinctual. However, when you take a bath, you have to clean your penis eventually and then poof : you just discovered your body kid. Then, if you are smart, you are gonna wank off a lot. Because it is fun. Then you are going to be told not to do it. For reasons you won't understand. And if you see everyone else doing it, you will wonder why can't you. I know the sole reason I wasn't groping the girls at school as a child was out of fear.
As for the "we don't want to see people naked part"
I work as a water game attendant. I make sure the area is clean and the water chlorinated properly. This is when I asked myself : why is it okay for children under six to undress outdoors, but not for adults, or older children?
I mean, come on, it's just a body. We've all seen what a penis looks like, and we can easily enough imagine what yours looks like. So why all that fuss?
Oops, that was rather an argument about people not wanting to be seen naked. XD.
Okay then.. well, if you don't want to see naked people... well, how silly of you, just don't look the time it will take me to put on my swimsuit. I hate running around without proper support for my testicles anyway.