The aim of the study is to discuss and problematise notions of femininity and masculinity constructed in teaching situations among 16-year-old upper-secondary students studying science. The empirical examples originate from a teaching session with the theme of "sex and relationships". The analysis is focused on metaphors inherent in a lesson that has its origins in the animal world. The findings show that the lesson "sex in the animal world" is full of anthropomorphism, metaphors that humanise animal behaviour. Teachers and students compare the animals' sexual behaviour with human behaviour, with the result that the animal world can be perceived as representative of natural sexual behaviour. The survey illustrates problems with how the examples are permeated by cultural values in the presentation of the animal world and how these examples form constructions of femininity and masculinity in the classroom.