I haven't had sex without a condom in ten years. But condomes apparently have a 3% failure rate when used "perfectly" over the course of a year. It's not the most scientific statistic but it still seems disturbingly high.
From what I've read about 40% of babies in western countries are un-planned. And that's just the people who admit it. And I don't think that that many people were just completely careless about contraception. It's just that no contraception is perfect. In Jurassic Park an all-female population procriated. With that in mind it seems far from unthinkable that even with more than one contraceptive something will slip through.
Life will find it's way.