Penis is a competitive beast
Scientists believe the shape of the penis may have evolved to help men remove the semen of love rivals during sex.
I'm usually pretty sceptical about all these evolutionary 'explanations' for just about any human characteristic you care to mention (especially those that mention sex), but this one does at least have a ring of plausibility about it: sexual organs are highly variable between most species, and are often the easiest way to tell similar species apart.