From Butler’s Gender Trouble 1990 p. 73
The incest taboo, then, would repress no primary dispositions, but effectively create the distinction between “primary” and “secondary” dispositions to describe and reproduce the distinction between a legitimate heterosexuality and an illegitimate homosexuality. Indeed, if we conceive of the incest taboo as primarily productive in its effects, then the prohibition that founds the “subject’ and survives as the law of its desire becomes the means by which identity, particularly gender identity, is constituted.