Doesn't the repositioning of modern contraceptive methods risk increasing the vulnerability of young people to STI/HIV/AIDS? This study was conducted using both a quantitative and qualitative approach. Based on a stratified two-stage cluster sampling, 570 young people from 19 public high schools in Cotonou and 100 young university students aged 15-24 from two universities in Cotonou were interviewed. Data collection was conducted between November and December 2016. A questionnaire was used to assess the knowledge and attitudes of youth on STI/HIV/AIDS and contraceptive methods. The study revealed that 62% of the respondents had their 1st sexual intercourse before the age of 15 with an equality between the two sexes. The frequency of sexual intercourse was at least four times a month for 72.3% of secondary school respondents and 93.5% of higher education respondents. Nearly 40% of secondary school respondents and 56% of those in higher education practice multiple sexual partnerships, with girls predominating.