I think you are making too much of this - remember the "stirrings" that indicate the onset of puberty can start from even 11 onwards, depending on how boys develop and at what stage this process begins. So I would agree with other editors here, especially with pohnpei397 - Jonas' dream is actually very innocent - it is not as if it is a graphic sexual description we are treated to.
Most 12 year olds are not subjected to as many naked bodies as Jonas was in The Giver. Remember, he spent much of his time bathing the elder inhabitants--the Old--in the bath house at the House of the Old. Although he was not allowed to look at "another's nakedness... the rule did not apply to newchildren or the Old." He spent many hours bathing older women, and he had a special rapport with Larissa before she was released. We can also assume that Fiona was naked while she bathed the old men, and though Jonas was not supposed to look, he probably did. Naked old women, naked old men, naked adolescents and babies--it's really no wonder that Jonas had dreams and "stirrings."
Just from my own personal experience, I do not think that is in any way strange.
We know that Jonas is somewhere around 12 (we don't know exactly because they don't do exact ages). This is not too young for boys to be interested in girls in that particular way.
You should notice that he does not dream about actually doing anything. That would be beyond his experience. So I don't think that his dream is (as you say) in "such volume."
To me the idea that a 12 year old boy would be interested in the idea of seeing a girl naked makes perfect sense to me. By that age, I was certainly interested, and I know my friends were too.
I believe that Jonas is already unique by the measure that he was chosen to serve as the receiver. He has become aware of new sensations and on top of that he has not been taking his medications.
Boys often beginning to experience a sexual awakening in their bodies around the age of twelve. In addition, he is also having sensory awakenings thanks to the new memories he has been given. A normal adolescent would already be experiencing new sensations, but Jonas' sensations have the added complement of all the other memories.