In answer to your question about why you might feel so uncomfortable with 'family' nudity when adult nudity doesn't bother you....I would think that, yes, your upbringing had everything to do with this uncomfortable feeling you get.
Do you remember your parents actually saying something about this topic? Were you caught in some 'nude' situation that brought reprimands when you were a child? Or was it just unspoken in your household.
I would almost bet that you can remember some specific event or words that contributed to the squirrely feeling you get.
Is it that you don't mind seeing children run around nude on beaches etc....but you just don't like them to see you nude?