[Sexual debut in adolescence and gender relations: a cross-sectional study in São Paulo, Brazil, 2002].

This cross-sectional study aimed to assess gender differences in adolescents in relation to onset of sexual activity. A total of 406 15-19-year-old adolescents from a family health unit in the city of São Paulo, Brazil, were interviewed from May to December 2002. Nearly half had already had their first sexual intercourse, at a mean age of 15, independently of gender. Sexual debut was usually unplanned (72.7%) and inside the home (86.1%). An equal proportion (61.0%) of males and females used some contraceptive method in their first sexual intercourse; however, more males had used a contraceptive method in their most recent intercourse. Having initiated their sexual activity in stable, affective relationships, females proved to be more vulnerable to STD/AIDS, since they switched from condoms to the pill in their subsequent sexual activity. Male adolescents continued to use mostly condoms, probably because they were involved mainly in non-stable relationships.