Research found that those who believe sexual orientation is inborn have generally positive attitudes towards gay men and lesbians. However, other studies have also found that these beliefs could include negative eugenic ideas. This study aims to investigate the role of people’s beliefs about the aetiology of sexual orientation on attitudes towards adoption for both gay and lesbian couples. We hypothesized that this relationship would be mediated by sexual prejudice. To test the predictions, 256 Italian heterosexual participants were asked to answer to a scale about their beliefs regarding the aetiology of sexual orientation, sexual prejudice, and attitudes towards adoption by same-sex couples. Results confirmed that the relationship between aetiology beliefs and adoption support was fully mediated by sexual prejudice. These investigation results suggest that the belief that sexual orientation is controllable may serve to justify one’s prejudice and, in turn, result in a lower support for same-sex couples’ adoption.