Long ago, in another plane of existence separate from Earth's, the elven population was seriously flagging. In order to bring the population back up to safe existence levels, a group of elven scientist/magigisters developed the Breeders. A species that has both sexes (hermaphrodites), with extraordinary capacity for reproducing, Breeders can carry babies, and father them on others. The gestation period for a Breeder carrying a baby is one week. After the child is born, it grows at an exponential rate, one year for every day. Once the elf reaches the apparent physical age of 15-18, the aging stops, almost altogether, and from then on the elf seems to age only one year per one hundred years. The scientists also built in a series of systems in the genetic makeup that ensure prolifigation. A Breeder reaching the age of reproduction will then go into a "heat," much like a dog in season. During the heat, the elf will emit an attraction pheremone, a scent, to attract potential mates. Also, the elf must have sexual intercouse. Inside the female parts of a Breeder's body is what is called "the second barrier", which must be passed, obviously by a man's sex. Once that barrier is passed, it releases a small amount of a vital hormone, a hormone needed by the elf in order to survive. The more heats that pass without satisfaction of the elf's need, the sicker the elf will become, until death occurs.
Note- While a Breeder can be somewhat effiminate, the general appearance is that of a male, so they are generally referred to as a "he".
This experiment succeeded, and the elven population was assured to survive. But once the elves didn't need the Breeders for that function anymore, things turned dark. The elves gethered up the Breeders, and essentially turned them into a slave race. Used as sex toys and made to reproduce until exhuastion, Breeders were not treated well.
And some escape that bondage.......