Breeder is a slang term (either joking or derogatory) used to describe heterosexuals, primarily by homosexuals. It is drawn from the fact that homosexual sexual activity cannot lead to reproduction, whereas heterosexual sex can, with implicit mocking by connotation of animal husbandry, the original usage of the word.
"Breeder" can also be used as a derogatory term by childfree people of any sexual orientation to refer to parents who focus on their children and abandon their previous friends and lifestyle, or to women who give birth to many children. The phrases "breeder, not parent" (BNP) or "parent, not breeder" (PNB) are used by some childfree communities to differentiate between positive and negative parenting.
The number of souls in this kingdom being usually reckoned one million and a half, of these I calculate there may be about two hundred thousand couple whose wives are breeders; from which number I subtract thirty thousand couples who are able to maintain their own children, although I apprehend there cannot be so many, under the present distresses of the kingdom; but this being granted, there will remain an hundred and seventy thousand breeders.
Swift's piece was meant as black humor, and refers to the breeding of children for cannibalism. Some parents resent being referred to as "breeders", and feel that the word unduly reduces the process of child-raising to animal husbandry.
The term was part of a 2006 controversy in the heavily gay resort town of Provincetown, Massachusetts, when petitioners against same-sex marriage whose identity was published complained of having been called "breeders".
Another slang term referring to heterosexuals is "egg packer", which mocks the connotation of the slang term for homosexuals, "fudge packer."