And the winner is...

A team of evolutionary biologists at the State University of New York at Albany released a paper in 2003 speculating the ridged glands of a man's penis are used to remove a competitor's semen before he deposits his own. Author Mary Roach, Bonk, writes that this makes sense given the last portion of a man's ejaculate has a natural spermicide used to kill a future donor's seed. 

--Mary Roach, Bonk, The Curious Coupling of Sex and Science