The present research tested the hypothesis that perceived men's feminisation can decrease heterosexual men's positive attitudes towards homosexuality because of their increased motivation to psychologically differentiate heterosexual men from gay men – i.e. in order to restore ingroup distinctiveness. Study 1 (N = 173) manipulated perceptions of men's feminisation and showed that prompting participants with bogus evidence that men are becoming feminine decreased positive attitudes towards homosexuality. Furthermore, the extent to which heterosexual men reported increased psychological differentiation from gay men (both at the interpersonal and the intergroup levels) mediated the impact of perceived men's feminisation on attitudes towards homosexuality. Study 2 (N = 178) used a fully experimental approach and manipulated perceived biological differences between heterosexual and gay men in order to threaten or grant ingroup distinctiveness. The results revealed that perceived men's feminisation decreased positive attitudes towards homosexuality in the distinctiveness threat condition (i.e. when gay men were described as biologically similar to straight men), but increased positive attitudes both when ingroup distinctiveness was granted (i.e. when gay men were described as biologically different from straight men) and when it was not relevant (i.e. when the similarity of all human beings was salient). We discuss the relevance of these findings for masculinity norms, attitudes towards homosexuality, and the ingroup distinctiveness literature.