Apparently, many narcissistic parents exhibit a disturbing disregard for sexual boundaries in their growing children. They are naked in front of their pubescent child of the opposite sex or walk in on them when they are changing. They bathe their children until a late age or shower with them. I don't believe family members being naked in front of each other is a bad thing at all provided it makes no one uncomfortable.
That appears to be the point for narcissistic parents. It's like they want to push the boundaries and make their growing children uncomfortable and get a protest from them ("Dad, I'm naked in here!") because they want to reply to that protest ("Oh, come on, you're my child, I changed you when you were a baby!") in a way that shows that the narcissist's child always remains the narcissist's child.
His CHILD. Not a budding, soon-to-be adult developing healthy boundaries.
HIS child. His property, an extension of his body.