Are children really born as 'pieces of white paper'?(or, why do we have sexual desires/instincts?)

I remember some article was saying internet porn has resulted in many teenagers being exposed to sexual materials too early. This got me thinking: are sexual desires innate and natural or unnatural(that we are all born like a piece of white paper and are then affected by external factors)?

For now, I stand on the side of 'innate and natural', because if we didn't, cave people, who do not have access to porn(soft or hardcopy) wouldn't have reproduced so many waves of descendants. However, let's say I conducted an experiment which involves a boy(assuming that he is innately straight) being enclosed in an all-male environment, with absolutely no exposure to any sexual materials, what conclusion would I get?

Let's look at this in a Christian's perspective(not a Christian myself but I shall borrow their theories in attempt for a different perspective on this). They say God created humans. So he probably played a part in inserting the element of having sexual desires into humans. Why would he do that? Are humans not smart enough to know that there is a need for reproduction to sustain our kind? Are humans not smart enough to find a way to reproduce through sex, without first having the desire to do so? The first two cave people who had sex probably didn't know that they were gonna produce a baby cos they were simply following their sexual desires/ instincts!

To be honest, I often start thinking about a question without knowing why so I don't really know where I'm going with this. Heck, I don't even have a conclusion! Or rather, I have the ironic conclusion of having no conclusion. I probably need to think more deeply about this but I'll just leave it here for now because I feel like it(again, an unexplainable instinct). Which brings me to another question: is it possible to have unexplainable instincts? Are instincts even what we define it to be? 

Or worse, are we defined by our instincts?