In the Western world, gender/sex is traditionally viewed as binary, with people falling into one of two categories: male or female. This view of gender/sex has started to change, triggering some resistance. This research investigates psychological mechanisms underlying that resistance. Study 1 (N = 489, UK) explored the role of individual gender identification in defense of, and attempts to reinforce, the gender/sex binary. Study 2 (N = 415, Sweden) further considered the role of individual differences in need for closure. Both gender identification and need for closure were associated with binary views of gender/sex, prejudice against nonbinary people, and opposition to the use of gender-neutral pronouns. Policies that aim to abolish gender/sex categories, but not policies that advocate for a third gender/sex category, were seen as particularly unfair among people high in gender identification. These findings are an important step in understanding the psychology of resistance to change around binary systems of gender/sex.