 Other reasons included a fear of sexually transmitted infections, a lack of desire, being afraid of getting caught, and the belief that sex was not appropriate for someone of their age.Epidemiologists at the Center for Disease Control emphasize that for sex education to be effective, it should take place before teens become sexually active.Both adolescents who have never had sex and those who have chosen to become abstinent after engaging in sexual behaviors cite the negative consequences of sex as reasons why they choose not to have sex.