Penis fencing is a mating behavior engaged in by certain species of flatworm, such as Pseudobiceros hancockanus. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes.
The species "fence" using two-headed dagger-like penises which are pointed, and white in color. The "winner" is the organism that inseminates the other. The sperm is absorbed through pores in the skin, causing fertilization in the "loser".
Child-bearing, while necessary for the continuation of a species, requires considerable resources from the mother. Thus, from a biological point of view, it is preferable to be the father rather than the mother. However, there are other hermaphroditic species where both partners try to be inseminated rather than to inseminate.