Abstract
At the upper secondary school level, adolescents seem to be very concerned about sex education programmes, mainly because of widespread information about AIDS, and, of course, because most of them are beginning their sexual life. Generally speaking, Mexican sex education teachers are dealing with two challenges; the steady increase of teenage pregnancy and the spread of sexually transmitted diseases (STD), especially AIDS. In observing the dynamics in sex education lessons and interviewing a group of teenagers; it can be concluded that to the subjects of this study, contraception decision-making, especially condom use, being more than a rational or a cognitive issue, is a matter highly determined by their sexual reputation.