Most often used in the derogatory to describe people that elect the suburban middle class lifestyle because they are so inept the only possible way they can contribute anything to society is to produce offspring. Typical cycle goes as follows: schooling -> job -> marriage -> children -> death. Their children also tend to grow up to become breeders and the cycle continues. True "breeders" can be compared to animals such as rabbits or cattle that do nothing but multiply by breeding. The term probably got its start in the GLBT community but has since gained outside popularity.

a straight person. a heterosexual.

Ppl who turn up at your house with children and allow them to write all over your books, wall coverings etc. This happens because they are jealous of your child free lifestyle which allows you to go out mid week and take long romantic weekends away while they are stuck at home trying to find a child minder/doing maths homework etc.

a noun.

someone whose sole purpose is to have babies.

They define their existence solely based on how many kids they have and how big their family is.

see also "heterosexual"

A heterosexual. A carbon footprint enhancer. That group of humans most responsible for global warming, deforestation and a depletion of the ozone layer. A propagator.

A heterosexual person. Formerly, this was an insult used by the gay community; it is now mostly used by straight people in a lighthearted vein to (mistakenly) show that they are down with gays and familiar with the culture.

Used by the homosexual community as a reference to "knuckle dragging" heterosexuals, meaning individuals (generally males) that care for little more then spreading their seed and getting into bar fights.

Not all heterosexuals are breeders.
A heterosexual's values and interests are as diverse as the world itself. A breeder's interests are narrow (beer and pussy), and conversation outside said interests are near impossible. And while the heterosexual is accepting of their homosexual brethren, Breeders are homophobic.