As everybody has said - nothing short of having your testicles removed, or your girlfriend having her ovaries removed is 100% (not even vasectomy or tubal ligation operations!).
Condoms in general are only 80% effective - this covers accidental breakages. BCPs are 98% effective - or so.
Go ahead and blow your load inside of her - just have a backup plan. Furthremore, because of STDs .. consider the trust/risk ratio
