Sex/gender is an interesting phenomenon in biological and social sciences. It is characterised by constant redefinitions and conflicts as knowledge increases and social norms change. To understand the change in sex/gender representations in biology education, we analysed Finnish secondary school human biology textbooks from the 1990s until the most recent books. We classified the depictions of sex/gender differences and their explanations in texts and illustrations. We also composed stereotypes of men and women and calculated the number of feminine and masculine words. Contrary to previous studies, this study shows that feminine words were more numerous due to reproduction being closely linked to the word 'mother'. Biology textbooks mostly outline biological sex differences and explain them with hormones. While we found fewer sex/gender stereotypes and inclusion of sex/gender diversity in the recent textbooks, there was a decreasing trend on discussing sex/gender. These changes can partly be explained by curricular changes and the emergence of an additional subject, health studies, which has led to emphasising binary biological sex/gender in biology. We suggest that our results shine a light on a process where 'queer' is partially included in hegemony but which does not question the hegemony through critical discussion and scientific knowledge.