The main purpose of this paper is to examine the content of first-grade textbooks to assess the way in which textbooks treat the focal points of sex education and to check their validity for use in coeducational contexts. The paper also attempts to provide school sex educators and teachers in general with ideas to stimulate reflection and analysis, to encourage them to read textbooks critically and to enable them to propose changes or even reject publishers who contribute in any way to the perpetuation of inequality between men and women. The paper presented here is a descriptive analysis of the mathematics and environmental science textbooks in Basque and Spanish (the two official languages of Navarra) most widely used with first-grade primary school children in the Autonomous Community of Navarra. The aim is to examine the way in which the textbooks deal with human sexual reality. A thorough review is made of the sex and gender content, both pictorial and verbal. The focus is on the treatment accorded to men and women and the traits assigned to each in both the text and the illustrations of the textbooks. The texts are also analysed to determine whether (and in what way) the basic elements of sex education, that is, the body, the family and friends, are depicted. Since the depiction of women is only 37.2% overall, the social role model insistently presented in the textbooks can be said to have the shape and form of a man; the role model therefore fails to reflect current social reality, i.e., that 51% of the world's population are women. The paper reports that, irrespective of the publishing house and the language in which textbooks are written (Spanish or Basque), the textbooks studied are a long way from achieving the sexuation of education, and stereotypes continue to be transmitted through textbook illustrations.