Form Follows Function...?

Possibly more than you wanted to know... or consider :

This piece from "Scientific American," reports certain evolutionary biology hypotheses and testing to account for the particular shape of the human penis (which differs significantly from our primate relatives)... a fair amount of conjecture, with a smattering of experimentation :

http://www.scientificamerican.com/article.cfm?id=secrets-of-the-phallus

A tad reminiscent of Robin Baker's book, "Sperm Wars."

There has been a small spate of popular books out recently on the never-ending favorite topic of human sexual behavior, including:

"Bonk" by Mary Roach, and "How Sex Works" by Sharon Moalem