The male anglerfish latches on to a female near her stomach area. He fuses his mouth to her body and proceeds to atrophy into a pair of gonads. During ovulating season, the gonads ejects sperm into the female. To stay alive, the male anglerfish feeds off what the female eats. Gives a whole new meaning to the word leech.

Also, flatworms engage in penis-fencing. Since flatworms are hermaphrodites, come mating season, they engage in a battle and the first to get pierced is actually turned into the female for the season. Battle of the sexes, anyone?