Guppies don't have a penis, as such. Instead, they have modified a pair of their fins into penetrating organs called gonopodia.
These penis-ish organs are tipped with an unpleasant set of claws, hooks ridges and spines. These features evolve very quickly and they're sometimes the only way of telling one guppy species from another. [...]
Compared to shaved males, clawed ones transferred three times more sperm into unreceptive females, but the same amount into those that willingly mated.
This strongly suggests that the claws are a "sexually antagonistic trait" -- one that benefits one sex over the other. In this case, they help the males to grasp resistant females.
Sometimes You Just Have To Shave Off a Fish's Genital Claws.