ABSTRACT Prior studies suggest peer group sex composition is an important correlate of delinquency, but these studies often conflate the influence of other-sex peers with that of romantic and intimate partners. This research seeks to understand the influence of other-sex peers as separate from, but related to, romantic activity and sexual debut. I find that while sexual and romantic activity predict mixed-sex peer group (MSPG) membership, they do not fully explain the positive relationship between MSPG membership and violent delinquency. Guided by literature on gender socialization and the institutionalization of heteronormativity, I argue this result can be explained as the result of a heteronormative culture that problematizes platonic, other-sex interactions and forces adolescents to maintain other-sex friendships in non-surveilled spaces.