From Wikipedia:

"Some anglerfish, like those of the Ceratiidae, or sea devils, employ an unusual mating method. Because individuals are locally rare, encounters are also very rare. Therefore, finding a mate is problematic. When scientists first started capturing ceratioid anglerfish, they noticed that all of the specimens were female. These individuals were a few centimetres in size and almost all of them had what appeared to be parasites attached to them. It turned out that these "parasites" were highly reduced male ceratioids. This indicates the anglerfish use a polyandrous mating system.

Certain ceratioids rely on parabiotic reproduction. Free-living males and unparasitized females in these species never have fully developed gonads. Thus, males never mature without parasitizing a female, and die if they cannot find one. At birth, male ceratioids are already equipped with extremely well-developed olfactory organs that detect scents in the water. Males of some species also develop large, highly specialized eyes that may aid in identifying mates in dark environments. The male ceratioid lives solely to find and mate with a female. They are significantly smaller than a female anglerfish, and may have trouble finding food in the deep sea. Furthermore, growth of the alimentary canals of some males becomes stunted, preventing them from feeding. Some taxa have jaws that are never suitable or effective for prey capture. These features mean the male must quickly find a female anglerfish to prevent death. The sensitive olfactory organs help the male to detect the pheromones that signal the proximity of a female anglerfish.

However, the methods anglerfish use to locate mates vary. Some species have minute eyes that are unfit for identifying females, while others have underdeveloped nostrils, making them unlikely to effectively find females by smell. When a male finds a female, he bites into her skin, and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. The male becomes dependent on the female host for survival by receiving nutrients via their shared circulatory system, and provides sperm to the female in return. After fusing, males increase in volume and become much larger relative to free-living males of the species. They live and remain reproductively functional as long as the female lives, and can take part in multiple spawnings. This extreme sexual dimorphism ensures that when the female is ready to spawn, she has a mate immediately available. Multiple males can be incorporated into a single individual female with up to eight males in some species, though some taxa appear to have a 'one male per female' rule.

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Finally, sexual parasitism may be an optional strategy in some species of anglerfishes. In the Oneirodidae, parasitized females have been reported in Leptacanthichthys and Bertella—and others that were not still developed fully functional gonads. One theory suggests the males attach to females regardless of their own reproductive development if the female is not sexually mature, but when both male and female are mature, they spawn then separately."

Would you like people to mate this way?