birth control is always 99% effective. if it happens, it happens.
and if you're really totally against any sort of post-conception contraceptive, you're going to have to start looking for adoptive services in advance. but knowing you, you won't exactly be on the top of the genetic pool they typically have people picking from, so you'll be shouldering a child in nine months. or you could just abandon your girlfriend like some people do and be a ratty non-committal twerp the rest of your life.