Acanthocephalan males have cement glands associated with the vas deferens, secretions from which seal the female vagina with a cap after copulation and so prevent subsequent insemination. Homosexual rape results in the male victim having the genital region sealed off with cement and effectively removed from the reproductive population. Sperm and cement are transferred to females during copulation, but apparently only cement is transferred to males during homosexual rape. Acanthocephalans conform to a parental investment model, and we interpret the evolution of the cement gland and sexual behavior as the result of sexual selection.