Abstinence works 100%... as long a you are abstinent.Actually this is an extremely unreliable form of contraception as it requires self-control that is beyond most people. Relying on abstinence is bonkers. It is also a denial of a major function of sex in humans, which is pair bonding. Delaying any sex until after marriage is a recipe for more failed and unhappy marriages.
To gauge the actual effectiveness of abstinence, multiply it by the proportion of people that actually manage it.