These days men and women are using condoms for three reasons: for birth control, to protect themselves against sexually transmitted diseases (STDs), and as a backup to other birth control methods to avoid slip-ups and accidents. All birth control methods (even sterilization), have a failure rate, so by adding a condom, you're more likely to get your effectiveness nearer to 100%.

"We really recommend two methods [of birth control] to have effective protection," says Anne Foster-Rosales, MD, chief medical officer for Planned Parenthood Golden Gate. "A condom is a great addition to any female method. It maximizes the pregnancy protection and also protects against STDs."

According to the Centers for Disease Control and Prevention (CDC), one in five Americans has genital herpes, and half of all sexually active adults will develop HPV. A national survey conducted in 2003-2004 found that 25% of teenage girls had an STD and nearly 50% of African-American girls did.

Many people who have STDs don't actually know they're infected. They may believe they're STD-free and tell you so, when in fact they aren't. If you and your partner are in a monogamous relationship, are using another form of birth control, and have both tested negative for STDs, however, you may decide to skip condoms.