During your teens you become sexually mature. If you're a girl, you develop breasts and begin to get your period. If you're a boy, your penis and testicles become larger. If you have sex, you could get pregnant or get someone pregnant. Whether you choose to have sex or not, it is a good idea to know about safe sex and how sex affects your health. Besides pregnancy , having sex puts you at risk of getting a sexually transmitted disease , such as herpes or genital warts, or HIV, the virus that causes AIDS. The only way to be completely safe is not to have sex. If you choose to have sex, however, latex condoms are the best protection against sexually transmitted diseases (STDs). Condoms are also a form of birth control to help prevent pregnancy.