After all, why in all of evolution would nature have designed a body part with such obviously enormous reproductive importance to hang off the body so defenseless and vulnerable? Although we tend to become accustomed to our body parts and it often fails to occur to us to even ask why they are the way they are, some of the biggest evolutionary mysteries are also the most mundane aspects of our lives.
Thus, the first big question is why so many mammalian species evolved hanging scrotal testicles to begin with.
Full article at Scientific American
Though I am in total disagreement with this: "the only human body part arguably less attractive than the penis--the testicles."
I rather like the look of both.