Here is a list of sites about sexual health and HIV made for pre-teens and teens. These sites are tailored mostly for Western audiences, so have a look first before recommending one to your own children.
Voices of Youth
This is the HIV page from UNICEF's Voices of Youth website. It is a comprehensive website for youth covering everything including how HIV affects youth around the world, prevention and treatment issues and real life stories of youth living with and affected by HIV. The website is available in French and Spanish as well.
Young People's Section: Avert
Avert provides a great site for middle school aged students. It is simple and straightforward, introducing and demystifying basic topics like puberty, sex, HIV, and also offering information about issues like sex and the law, birth control, relationship advice.
SEX, etc (a website by teens for teens)
This site offers resources galore on almost every issue regarding emotional and physical health for teens, including sexual education, abuse, emotional health, alcohol and drugs, and basic girls’ health and guys’ health.
A great sexual health and education site for all teens. The site is colorful and engaging. In addition to basic information, it provides anecdotes and offers a section on sexual politics. The site may take a longer time to download, due to its high visual content.
Go Ask Alice!
Columbia University’s Free Health Question and Answer Site is a great resource for teens.