Penis shape evolved to scrape other mens semen out of the vagina.
'"They found that the coronal ridge... could scoop out more than 90 percent of the cornstarch mixture (from the artificial vagina) with just one thrust, while a phallus with no coronal ridge only managed to remove 35 percent," New Scientist magazine said on Wednesday."
http://story.news.yahoo.com/news?tmp...ealth_penis_dc