Postcopulatory sexual selection in internally fertilizing animals can select for harmful male genitalia, which injure mates during copulation and affect female fitness. Harmful male genitalia may further affect fitness of heterospecific females given interspecific copulation, particularly during secondary contact. Male Callosobruchus seed beetles have spines on their genitalia, and C. chinensis males reduce the fecundity of C. maculatus females via interspecific copulation, of which underlying mechanism is unknown. By exploiting phenotypic variation among nine separate C. chinensis strains, we found that (i) the more exaggerated the genital spines of C. chinensis males were, the more likely they were to induce genital injury in C. maculatus females, and that (ii) the genital injury translated into fecundity loss. That is, genital spines affected heterospecific fitness via interspecific copulation and genital injury. Harmful male traits due to sexual selection may generally affect the extent of interaction between closely related species.