Centers for Disease Control and Prevention (CDC)
http://www.cc.gov/
Find information about sexually transmitted infections (STIs) here.

Coalition for Positive Sexuality
http://www.positive.org/
Nonprofit organization for teenagers.

Intersex Society of North America (ISNA)
http://www.isna.org
Advocates for intersexed children and their parents.

Sexuality Information and Education Council of the U.S. (SIECUS)
http://www.siecus.org/
Offers information and education about sexuality.

Sexuality Library
http://www.sexuality.org/

Society for the Scientific Study of Sexuality (SSSS)
http://www.sexscience.org/
Offers information on the SSSS, dedicated to interdisciplinary research into sexuality.