Secondary school students' early choices related to staying in the science track define their future decisions to choose chemistry at college. This investigation aims at analyzing the role of gender in students' causal attributions to choose or abandon chemistry when it first becomes optional in the Spanish educational system. Our analyses uncovered a relevant effect of gender in the students' decision, boys being more likely to choose physics & chemistry when they face, for the first time, the possibility of continuing or opting out the subject. Besides, students' causal attributions to the subject relationship with mathematics and to friends are affected by gender regardless of the students' level of motivation. In turn, there is a gender effect in attributions to friends and media only in the case of highly-motivated students. A multinomial logistic regression model revealed that gender is a strong predictor of the students' decision. The regression model also uncovered a significant interaction effect between gender and attributions to the subject relationship with mathematics, girls becoming less likely to choose physics & chemistry when the latter increase. Our results highlight the need of working on the students' and families' stereotypes and propose gender-balanced teaching models to close the gap between girls' and boys' attitudes, motivation, and anxiety towards mathematics in the context of physics & chemistry teaching and learning.