Women are not able to fall pregnant 25-50% of their cycle. The ovum has a 24 hour window to be fertilised and sperm may survive for 5 days. That's 5 days of an average 28 day cycle.
You say the only effective way of preventing pregnancy is abstinence... Then go on to say 'there is always a failure rate sometimes due to the mechanism, other times due to failure to use as proscribed'... Well abstinence is quite often not used as proscribed, and is therefore not effective (see PEARL index comment above).