It's called birth control, because someone controls the birth. When you don't put a condom on, she controls the birth.
Presently there are 47 different commercially available methods of birth control for women; there are 2 for men - vasectomy and a condom. With that many options of birth control for women one would think that there would be a dearth of pregnancies in the past 30 years, instead we've had the largest statistical spike in "unintended" pregnancies, and the largest incidence of single parent homes (overwhelmingly single mothers) since records were started. Make no mistake, women who become pregnant want to be pregnant.
There are no 'accidental' pregnancies anymore. Always operate from this assumption. There are far too many ways to prevent pregnancy for this not to be the case. In addition to this YOU allow it to happen by not taking steps to control the birth yourself. I realize it's difficult to objectively consider this while you're in the midst of hot monkey sex with a girl - especially if you've been sexually deprived for any length of time - but you need to look at her from the default perspective of her WANTING to become pregnant. That may or may not be the case, but it's the presumption you MUST operate from, in spite of yourself, your emotions, attachment, etc.