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Why is it that Africans who grow up in a country where everyone is naked aren't all seeing therapists? How about Danes? Danish people love being naked with family members. Japanese? They take family baths together. What about that handsome nudist family in the soap commercial, are they irreparably damaged?
The tiniest bit of common sense will tell you that children will not be scarred for life by seeing adults naked. The psychologist Cary links to says, basically, "There's nothing wrong with it but if you think there is I'm okay with that because you Puritanical morons pay me. Whatever. Raise your children however you want, just don't mail me any more death threats."
The only thing to be concerned about is the crazy people around you. You don't want your son innocently saying something that will get your children taken away. So try to be aware of the standards of the people around you.