Abstinence is not the best choice, It teaches young kids in high school and college how to not have sex. Yes it can work to not have pregnancies, and catch disease but it is far from the best choice.
Abstinence most of the time teaches that it is the only way to not get pregnant or catch an STD. However, contraceptives are reliable. Birth control is also something that almost alway prevents pregnancy, and accidents do happen all the time but there is the Plan B pill or the morning after pill if you forget to take birth control or use a condom.
Sexually Transmitted diseases can be acquired, even if those participating are not impregnated. Abstinence is a choice but if you take PrEP which is a once a day pill like a birth control it has proven to almost completely eliminate the chance of getting HIV. Some STDs will go away with treatment, but what many don't know is that many of them linger in your system, and once inside your bloodstream can occur again, or be passed on to others. WHich is why getting tested and open communication with partners is best so you can discuss together what precautions you want to use.
Many young people also do not discuss past sexual partners which can lead to a slew of onset diseases. Which is perfectly fine not to discuss partners, but getting tested and talking with your partner is very important.
Abstinence teaches you not to have sex, but it doesn't discuss how to say no to a partner or how to talk with a partner openly about your body or how to find what you want in a sexual partner. Sex has benefits, so teaching abstinence isn't the best choice, but it is a choice.