Caring and nuturing a newborn is a lot of work and in our society costs money. Babies who do not have proper care can easily turn into detriments to society. There are not enough people willing to adopt. There at some time needs to be a halt to population growth or our quality of life will deteriorate and the death rate will rise. We have the ability to control our reproduction.
Abstinence is the most effective means of birth control.
Look at the possibilities on page 663 and their effectiveness. Tubal ligation or vasectomy require surgery and are not always reversible. You can't well change your mind. Works by not allowing eggs or sperm to be released. Works by preventing ovulation.