There is nothing more simultaneously ineffective and effective at prevention pregnancy than abstinence. 

Abstinence as an action in the moment is the most effective method of contraception, this is not something anyone argues with.

But abstinence as a policy for long term birth control is the least effective way of preventing pregnancy. People want to have sex, full stop. 

All humans are biologically driven to breath, drink water, eat food, and have sex to reproduce.

People are recommended not to eat unhealthy, yet the physical drive to eat that is taken advantage of by all the processed foods give us a society with pandemic levels of obesity and diabetes.

Similarly recommending people to just not have sex, is not effective at preventing it. 

The data indicates that abstinence only education causes more unwanted pregnancies and abortions than it prevents.