What you wrote in your first reply was absolutely right,
I think being a 9yr-old, when i feel respected and comfortable with the man and his nakedness and my privacy is not intruded it may even be fun.
Let me be clear: I don't want to give any man an excuse to force a boy to showering with him.
I agree that it's not the act of father and son being comfortable together, naked in a shower that is the problem.
the problem comes when one of them feels uncomfortable, especially the boy. At that point power and influence become a problem as well, and for me certainly the abuse of power my abusers had over me had a worse effect than the sex.