Let's assume condoms are 99% effective (which they're not), that means you'll get get a baby on average every 100 times you have sex. If you have sex everyday, you'll get your girlfriend pregnant over 3 times a year! Now of course that's absurd, but even if you had sex once a month, during ovulation, that's still an unwanted pregnancy every 10 years on average. Even if it were 99.9% effective, that mean out of 10 women using condoms would get pregnant every 10 years.

Birth control isn't fail proof.