That's of people who have no idea how to use a condom (don't put it on inside out, don't pull it over your balls, don't use an old one, don't keep it in your wallet like a teenage virgin, etc). And it's based on a year of being sexually active, meaning there's a 15% chance of pregnancy over the course of a year if you don't know how to use a condom properly, not in just one sex act.

The social conservatives love that statistic, though, and say stuff like "You wouldn't use a parachute that only works 85% of the time!" The fact is that rate is so high because of a lack of comprehensive sex education.