In a comment about condoms, DANEgerous points out:
In contrast condoms break, or slip, up to 12% of the time. That may not sound like a big number but if you have sex more then once, each event is cumulative and the probability of unsafe sex quickly exceeds 100%. Given the average single person 'hooks up' with a few partners a year, a few times each, multiply those dozen "trials" by a failure rate that high and the average single person exceeds 100% every year.What school did you attend, dude? Probability never exceeds 100%. Didn't I teach you that? Repeatedly?